I made this exact upgrade about 6 months ago when the prices first started dropping in Canada last year. I had the 550D for about a year and a half before making the change.
I'm very happy with the upgrade. With regard to your specific question about autofocus I find the cameras fairly equivalent. I've seen an increase in the keeper rate, but my experience has been that I still miss a bit more than I would like. I've seen this with things like birds in flight and running dogs. You'll probably also see an improvement, but don't expect a night and day difference.
The brighter, larger viewfinder probably contributes to the higher keeper rate in my case. (Hey, who knows, maybe it's just due to my dogs getting older?) I never thought the viewfinder was an issue with the 550, but now every time I pick it up I find it frustrating.
I've found that I can live with the autofocus on fast moving subjects just fine. My bigger issue has been with finding focus in darker areas. I can't say for sure that the 5D is worse than the T2i, but it certainly isn't any better.
I'm using the 5D with a 24-105, 16-35, 70-200 f4 IS, 50 1.8 and 85 1.8. I'm extremely happy with all of those lenses. I kept the T2i when I upgraded, but sold my 17-55 and Sigma 10-20 to move up to full frame lenses. I miss the 17-55, but either the 24-105 or the 16-35 make great walk-around lenses depending on the situation and my mood.
I use the 5D anytime I'm specifically going out to shoot, but I keep the T2i with me pretty much all the time now. It almost feels like a point and shoot to me now that I have the 5D (especially with the 50 1.
. I guess it's all relative.
Fire away if you have any other questions about the upgrade.