the tamron 17-50 wouldn't be mutch wider then the kit lens the t2i comes with. Did you buy only the t2i body?
This lens the sigma 10-20mm is a very wide angle lens and whats good about it is you don't get the fisheye distortion
http://www.amazon.com/Sigma-10-20mm-4-5-6-Digital-Cameras/dp/B0007U00XK
If you do want the fisheye distortion then i would go with the Rokinon 8mm fisheye
http://www.amazon.com/dp/B002LTWDSK/?tag=hyprod-20&hvadid=15469392579&hvpos=1o1&hvexid=&hvnetw=g&hvrand=21225699881027805887&hvpone=&hvptwo=&hvqmt=&ref=asc_df_B002LTWDSK
I have the rokinon 8mm for my t2i and love it. Its extremely wide and can see about 160 degrees around
check out this edit I made with it, all my other videos filmed with the t2i that arnt a fisheye were just shot with the kit lens 18-55mm
http://www.youtube.com/watch?v=PnhK622w8Fc&list=UUsFTfTZuj0yX1ahjVyMVvPA&index=2&feature=plcp