This is going to depend very much on what camera body you are using. Canon cameras do not focus past f/5.6 unless you are using a 1-series body (or are willing to do some warranty-voiding pin taping to force f/8 AF on unsupported bodies...which is usually a moot exercise anyway). Slapping a 2x TC onto a 70-200 f/4 is going to give you an f/8 aperture, and you are most definitely going to lose AF on anything other than a 1D something. Given that you are on a 30D, the AF speed will be a perfect zilch with the 70-200 f/4 + 2x TC combination, which is pretty much a no-go for wildlife and birds.
I'm a die-hard bird/bif/wildlife shooter myself, and I currently use the 100-400 f/4.5-5.6 L IS. Its a great lens, has decent IS, and is pretty easy to carry around for a few hours at a time (or pretty much all day with the right kind of strap, such as a BlackRapid R-Strap...or better yet, a more durable home-made version of the same.) The IQ of the 100-400 is great. The lens is definitely getting a bit dated...the IS mechanism is good up to about 2 stops at most, however when it comes to birds, its maybe good for 1 to 1.5 stops at best. Canon is rumored to be working on an f/4-5.6 replacement with 4-stop IS, which would be a significant improvement to any replacement...however expect it to come with a heftier price tag ($2000+ vs. the current f/4.5-5.6 $1600 street price).
For wildlife shooting, the 100-400L is superb in pretty much every respect. You don't need as much IS for hand-held shooting of wildlife as you do for birds/bif. It offers good reach with a modern high-res cropped sensor (it offers an FoV effective 160-640mm), however with the 30D's 8.2mp sensor, the benefit of extra reach is not really going to be as great as if you were using a 600D/60D/7D with their 18mp sensors (roughly 75lp/mm vs. 113lp/mm spatial resolution.)
You could certainly try to manually focus for wildlife...and so long as what your photographing is fairly slow-moving, you could probably get away with that on a 30D. Its lower spatial resolution means its not going to be capturing fine detail to start with, so it will be more forgiving of less precise focus. For anything that is moving that you will want to track, AF is pretty much essential. To that end, the 70-200 f/4 + 2x TC is a non-starter, especially if you were considering the non-IS version. For hand-held tracking, both IS and AF are pretty much given essentials for wildlife and birds, so the 100-400 f/4.5-5.6 L IS is really the only option here.
There are also other options you could look into if you can expand your budget a bit. A 300mm f/2.8 L or 400mm f/2.8 L, IS or non-IS, with a gimbal-type tripod head and a sturdy tripod, offer a LOT of flexibility with TC's. An f/2.8 lens can take a 2x TC and still be only an f/5.6 aperture, so it will still AF on any Canon body. These are certainly more expensive than the $1800 100-400 L, but older used ones can often be found on eBay and Craigs List for around $3000 to $3500 (original non-IS versions), and starting around $3500 to as much as $6000 (IS versions). A 300mm with 1.4x TC is 420mm, while with a 2x TC is 600mm. The 600mm reach is pretty much the ideal sweet spot for a LOT of birding, as it lets you frame birds pretty tightly without encroaching their comfort zones. A 400mm with 1.4x TC is 560mm, and with a 2x TC is 800mm. At 560mm you have some great reach and excellent IQ to frame wildlife at a safe/non-intrusive distance. At 800mm you can frame wildlife and bird portraits without intruding into safe/comfort zones. All the while, any of these focal lengths would be at an f/5.6 or f/4 aperture, and plenty capable of supporting AF on any Canon body.
If you have the funds, a older 300mm f/2.8 with a 2x TC could offer a lot more than the 100-400, greater reach with better IQ.
PhotoSE is currently running a "lending library" of sorts that allows our members to try out gear rentals and be reimbursed for it, at the cost of having to ask questions related to the gear or write a few blog entries on the PhotoSE blog about your experiences with the gear you rent. You could use this offering to try out the 100-400 to see if would work for you, and if you were willing to chip in a little money and some extra time writing more blogs, you might even be able to try out a 300mm f/2.8.