The AI-Servo tracks better on the 7D. I don't know if it really focuses any faster, but it either estimates better, the lower shutter lag improves the estimates, or the firmware doesn't intentionally cripple it for a low-end camera. I'm not sure which.

According to Bryan, the RC-6 should work on the 7D.

You'll lose the T3i's video zoom feature, if that's something you care about.

I would hesitate to get the 28-135mm. 28 is not wide at all. The 11-16 is ultra-wide, so you'll still be missing an important chunk of focal length, and you'll constantly be fiddling around swapping lenses. If you're looking for a quality replacement for the kit lens, with more range, and you're fine with the aperture on the 18-55mm, the 15-85mm is the lens to look at. For a wider aperture, look for the Canon 17-55mm IS, or look for a Tamron 17-50mm (non VC) to save a few hundred.

From what I've seen in reviews, when it first came out, the original 18-135mm (non STM) isn't anything special. I'd rather take any of the three lenses I mentioned above. The 18-135mm STM looks to be decently sharp, but I haven't yet seen an in-depth review of it.