First things first: If you're going to use a Kenko 1.4X extender get the 7 element Teleplus Pro 300 DGX rather than the 4 element MC4. There's usually very little difference in price. There is a perceptible difference in quality.
I own the Kenko C-AF 2X Teleplus Pro 300 DGX and the Canon EF 70-200mm f/2.8 L IS II lens. Honestly, the only thing I've found the TC good for is taking photos of the moon. For most other uses, I find I get better image quality shooting with the lens alone and cropping the snot out of it when editing.
Second: The light penalty of using a 1.4X TC will be the same with any lens - one stop.
- The EF 100-400mm f/4.5-5.6 L IS II will become a 140-560mm f/6.3-8 lens
- The EF 70-300mm f/4-5.6 IS II will become a 98-420mm f/5.6-8 lens
This will not only affect AF speed, it may affect the ability to AF at all. The original 7D is only rated to AF with lenses f/5.6 and wider. In my experience, the 7D will not successfully AF with an f/8 lens + TC combination unless pointed directly at a very high contrast object such as a bare light bulb, but it will try to AF with such a combination. With the EF 70-300mm f/4-5.6 plus a 1.4X TC, you'll be somewhere between f/5.6 and f/8 over most of the zoom range.
Many users report that third party f/6.3 lenses will AF with their Canon bodies rated at f/5.6. It seems the f/5.6 rating might really mean "anything wider than f/8." It may mean that older Canon bodies rated for f/5.6 aren't firmware limited to disable any lens + TC combo slower than f/5.6 the way newer Canon bodies are. Or it may mean that the third party lenses are reporting to the camera that they are f/5.6 lenses manually set at f/6.3.
When the EOS system was developed in the 1980s there were provisions to allow using lenses with manual aperture rings to report two values: the maximum aperture and the current aperture. The earliest EOS TS (tilt/shift) lenses used manual aperture rings on the lens. The newer TS-E (tilt/shift - electronic) lenses have an electronically controlled aperture, but every EOS body ever made by Canon is supposed to be equally functional (within the lens' designed constraints) with every EOS EF/TS/MP lens ever made by Canon. So the system still allows a lens to report two values for maximum and current aperture.
Even the very best glass Canon makes, such as the EF 300mm f/2.8 L IS II, on the very best body Canon makes, the 1D X Mark II, slows the AF a bit when a Canon 1.4EX III is added to the mix. Using a Kenko TC with a 7D will slow it a bit more (I've used the Kenko 2X with a 7D and 70-200/2.8 in the past). I have not used the EF 70-300mm f/4-5.6 IS II with any type of TC/extender. Just as the non-L lens will AF slower than the EF 100-400mm f/4.5-5.6 L IS II when both lenses are bare, my strong hunch is it will also AF slower when both lenses are attached to a 1.4X TC if you are using a body that can focus at f/8.¹ It goes without saying that when compared to a bare 100-400 the slower 70-300 will be even slower with a TC.
¹ Current Canon bodies that can officially focus at f/8: 1D X Mark II, 5D Mark IV, - up to all 63 AF points with v.III extenders and certain lenses; 1D X, 1D C, 5Ds, 5Ds R, 5D Mark III, 7D Mark II - center AF point with surrounding 8 points as 'assist' points; All other 1-series bodies - center AF point only; 6D Mark II, 80D - center AF point only except 27 AF points with 100-400 II and 200-400. No other Canon bodies officially support AF at f/8 or narrower. Many models older than about 2011 will try to AF with such combinations. A few, such as a 5D Mark II + Kenko 2X + 24-105/4 at 210mm/f/8, will even succeed.
EF 100-400mm f/4.5-5.6 L IS II
It's a very new design that has received very good reviews. Like any 4X zoom lens, it is not perfect. But as 4X zooms go, it is very, very good. Unlike most telephoto zoom lenses with a more than 3X zoom ratio, it does not soften noticeably at the long end of the focal length range.
EF 70-300mm f/4-5.6 IS II
This lens is a pretty good lens for the price at which it sells. But it sells for basically one-quarter the price of the 100-400 "L". While not terribly soft, it's not impressively sharp either.
In his review of the EF 70-300mm f/4-5.6 IS II, Bryan Carnathan says this when comparing both it and the EF 70-300mm f/4-5.6 L IS to the EF 100-400mm f/4.5-5.6 L IS II:
Many cheers went up when the Canon EF 70-300mm f/4-5.6L IS USM Lens was introduced and it has long been a much-loved lens. The L lens has a better build quality than the IS II, including weather sealing, but downsides are its heavier weight and considerably higher price tag. Though not terribly far off, the 70-300 IS II does not reach the L's level of optical performance. The L has a 1/3 stop wider max aperture over some of the range and the IS II has a higher MM (0.25x vs. 0.21x).
Performing at a much higher level than both of these lenses is the Canon EF 100-400mm f/4.5-5.6L IS II USM. However, this lens is in a different class. The 100-400 L II is an incredible-performing lens and is significantly sharper than the 70-300 IS II over the entire shared focal length range. All will prefer the 100-400's image quality, many will find giving up the 30mm of range on the wide end worth gaining the 100mm on the long end and none will prefer the higher price, the heavier weight or the larger size of the L lens.
As his image quality comparison between these two lenses shows, there is a marked image quality difference between these two lenses. You can change the focal length and aperture of each lens to compare them at various focal length and aperture combinations. At 300mm the 100-400 is sharper wide open at f/5.0 than the 70-300 wide open at f/5.6. The 100-400 is sharper at 300mm and f/5.0 even when the 70-300mm is stopped down to f/6.3 or f/8!
The build quality and durability is also much better for the "L" lens. It's heavier and larger as well.
Please also give your personal opinion if the price difference is worth it.
Each person has to answer that for themself. The difference might be worth it to one person and not worth it to another. The money saved on the difference could buy a couple of nice prime lenses or another very nice zoom. If the only lens you need/want is a long telephoto zoom, then having enough money left over for an 85/1.8 and a 135/2 or a 16-35/4 doesn't do you any good.
On the other hand, for some of us there's no substitute for the best image quality we can afford. It all depends on how much you are willing to spend to get there.
I own the EF 70-200mm f/2.8L IS II. It is the best zoom lens I have ever owned. I had to save for quite a while to be able to buy it back in 2010. Many meals that could have been eaten in restaurants were cooked at home. Many other things I wanted were put on the back burner. The cost of this lens was totally forgotten when I looked at the first images I shot with it. To me it is worth every penny I paid for it. I consider it some of the best money I have ever spent on anything.
Whether that is true for you depends on what you need and expect out of a lens, and how much you are willing to pay for it.
In addition to the EF 100-400mm f/45-5.6 L IS II or EF 70-300mm f/4-5.6 IS II, there are a few other options.
Both Sigma and Tamron make 150-600mm f/5-6.3 stabilized lenses. Both the Tamron and Sigma Contemporary lenses have been well received by many hobbyists that shoot in daylight. Both are compatible with the maker's respective USB lens dock that allow the end user to perform firmware updates and detailed AF calibration without sending the lens to a service center.
The Canon EF 400mm f/5.6 and EF 300mm f/4 primes are economical older designs that are very good optically. Neither has IS, though, and obviously neither can zoom.
Sigma has also recently introduced a 100-400mm f/5-6.3 as part of their Contemporary group within the Global Vision lens series.
The older pre-Global Vision series' 50-500mm, 80-400mm, 120-400mm and 150-500mm offerings from Sigma weren't on the same level as their recent Art, Sports, and Contemporary lines. Particularly at the longest focal lengths, they tend to be fairly soft.