The 5DIII would be much better than the t2i in low light. Having moved from a crop body to a 5DII, I find that I have a 1-2 stop noise advantage with the 5DII (LightRoom processed RAWs), and the 5DIII is better than the 5DII.
If you can wait, don't buy the 5DIII until the after the 24-70II comes out. If you're happy with the IQ and shutter speeds with your t2i, then wait for 24-70II. If you find that you'd prefer faster shutter speeds then, I'd suggest looking at a 24 or 35mm f/1.4 prime if you can handle the thinner DOF. I'd avoid getting the 24-70 I or 24-105 as stop gap measures. Keep using your current setup for the meantime.
My initial post was written with the assumption that I would sell the crop in favor of the FF. However, I think I'm just going to keep both bodies so that changes my needs a bit. I probably won't even get the 24-70L II since I already have the 17-55 for the crop, and there would too much FL overlap.
Since I'm keeping the 17-55 on the crop most of the time, the only decision is what to put on the FF.
1) 50 1.4 / 85 1.8 / 135L
I will probably use one of my existing primes (probably the 135) and see if the FL fits my needs. If it doesn't, I can work my way down to the 85 if need be.
2) 16-35L II
I was thinking of getting this for the FF but that would be as pointless as getting the 24-70L II because there's too much FL overlap with the 17-55 on the crop. The only way I would get this for the FF is to have that UWA and then put one of my primes on the crop body for the reach. But I think the costs outweigh the benefit in this case.
3) 70-200L IS II
I was thinking of getting this for the FF but I'm wondering if it's too beastly to carry around with me along with another body. Walking around a venue, bumping into people. I would have the entire 27-200 FL covered though, which is nice.
I'm leaning towards keeping the 17-55 on the crop and using one of my primes on the FF. The reason being is that I'm not quite sure what FL I would need on a second body as of yet. Also, I only have one flash, the 580EX II. I'm kind of wary on using the 70-200L IS II on my FF without a flash because I'm just not sure how 2.8 will do for low-light event type stuff. That and the whole beastly nature thing lol