I borrowed the 17-55 once for a short time. I found it heavy, flimsy build, and dicey AF. Maybe I was too focused on those to notice the IQ. Do you think it is superior to the 16-35 on a crop ?
I do think it's better. The 16-35mm definitely has better build quality and a nicer feel. As for weight, the 17-55mm and the 16-35mm II weigh essentially the same, so if you found the 17-55mm heavy, the 16-35mm will be heavy, too.
As for dicey AF, what do you mean by 'dicey'? With fast lenses (f/2.8 and faster), systematic AF errors are more obvious - those occur when your body and your lens are not well-matched (e.g. body is toward one end of the manufacturing tolerance, lens is toward the other). Your Tamron may be a good match, the 17-55mm you tried may not have been. AF Mircoadjustment is a feature that allows you to calibrate your lenses to your camera(s), and that was one of the main reasons I upgraded to the 7D. So, for example, my 17-55mm on the 7D is at +1 (scale is -20 to +20) - that small an adjustment, you'd likely not even notice a problem. But my 16-35L requires a +9 adjustment on the 7D; one unit is 1/8 of the depth of focus, so an adjustment of 9 units means what you focus on may well end up outside the DoF, especially with a close subject at the long end of the zoom. So...since your T2i lacka the AF microadjustment feature, I'd recommend that whatever lens you buy, you get it from a dealer where you can easily exchange it for another copy, or from a brick-and-mortar store where you can test the specific lens on your camera.
