The tamron 28-300 is a fascinating and frustrating lens. It doesn't go wide enough (on a cop sensor camera like the 500d or 7d, I'd want the wide angle for landscape to be more liky 15-17 and not 28; it makes me want to add the sigma 10-20 to my arsenal). And at the telephoto end, it goes soft like most super zooms do. I try not to use it much past 28-150 except in an emergency. This week I had that emergency (ran across a bear in yosemite with the "wrong" lens in hand) and realized just how much I wish I'd had my other lens available. Oh well.
But it's a perfectly nice street lens, especially for what it is, once you learn its weaknesses and work around them.
Having said that, for landscape, 28mm isn't wide enough, and 300mm isn't telephoto enough for wildlife or birds. IMHO. You will find that no matter how powerful a telephoto you buhy, you'll run into situations where you want more. There are days when 800mm isn't enough. This implies that as you progress, technique and planning become as important, or more important, than raw lens power. So don't overbuy lenses expecting it to solve things magically, it won't.
300mm, however, isn't powerful enough for most bird photography and most wild animals. 400mm on a crop sensor can be used very successfully. I use the canon 100-400 a lot; I use the canon 300/F4 + 1.4x tele some and I'm leaning on that combo more because it's sharper than the zoom but not as flexible; basically, I use the 100-400 handheld, the 300+tele mostly on a tripod. Either will run you roughly $1500 out the door. ($US).
Art Morris his shifted to using the canon 70-200 F2.8 IS USM (you want IS) with a 2x Tele instead of the 100-400. That's a big hunk of glass, it's much sharper, and it'll run you twice the cost of the 100-400. But that combo is very powerful and flexible and good, and it's something I'm considering experimenting with, not that I can afford it any time soon.
I've read a number of reports on the sigma 50-500 indicating it's a good lens, but like most megazooms, goes soft at the top magnification. You should rent and test these lenses before buying them, to see if they're acceptable. some photos I've read think it is, others don't. That's really true of any lens: rent and test before buying.
You won't cover all the ranges you want happily with one lens. Maybe not two. And lenses bigger than 300mm get expensive, although the canon 400 F4 is surprisingly affordable amd would probably tolerate a 1.4x tele on it fairly well. I haven't tried yet.
If getting closer is your primary wish, then I'd suggest looking at the 400F4. It's a bit more expensive than the Sigma but it'll be a much higher quality lens. Add in a 1.4x tele at some point to push it further. Or plan to upgrade your lenses over time. I have as I've gotten better at this moved away from zooms that cover larger ranges in favor of higher quality zooms that don't zoom as much, but are sharper and crisper and faster. Faster gives you options in the field to shoot in poorer llight or use teleconverters to leverage your lens collection.
In a perfect world, the Sigma 10-20, Canon 24-70 F2.8, Canon 70-200 F2.8 IS USM, Tele 1.4, Tele 2x and the Canon 400 F2.8 would make me happy. That's ONLY $4000 US in glass, not including the $8000 US for the canon 400 F2.8. Here in the real world, not gonna happen. and I haven't asked for a macro, fisheye, or tilt shift yet... (grin)
But with the lens you have, the Canon 400/F4 adds range. Over time, you can add in a better wide angle (canon 15-85, maybe?) and then the 70-200 F4. Not sure I'd use that with a 2x tele, but the 1.4x would cover your rang up to the 400. and the 1.4x on the 400 pushes you out over time from 15-500mm coverage with reasonably fast, sharp glass. That's still $4000 US, but it's a lot better than $12,000 US for being about a stop slower across the range. Going from F4 to F2.8 is expensive.
One other option to consider: upgrade your body. the Canon 7d sucks in a lot of pixels. One thing that lets you do is CROP without losing your detail, and you still keep the crop sensor which is useful for leveraging your glass for wildlife. So instead of pushing more powerful glass, get a more powerful body that captures more pixels so you can crop them away and still have usable images. or do some combination of all of this. Over time. Remember, buying GOOD glass over time is a nice investment because lenses should last you a long time if cared for, where bodies are going to get upgraded more often. So it's usually better (IMHO) to buy better glass and lesser bodies, or wait a bit and buy better lenses later than a cheaper one now...
