I own a Canon T2i and have the stock lens which is 18mm - 55mm. When I was in Italy I wasn't able to get back far enough to capture everything I wanted. Would purchasing a Wide Angle lens help in that situation or is a 18mm-55mm stock lens as good as it is going to get?
Actually the limiting factor is likely to be the T2i. That's why with still and now video I've always stuck with the full frame Canons like the DS1, Mark I and II and now the 5D. I believe that on a sensor the size of the T2i at 18mm you are getting the equivalent of a 28mm lens on a full frame camera, so semi-wide but not wide-wide.
One of my favorite lenses is Canon's 16-35mm, f2.8 zoom. I favor wide angle shots for the kind of industrial work I do. The downside to this lens is the cost. Around $1,650 I believe, but Canon has a mail-in rebate of $200 on this and several other items until July.
If you're testing the lens that came with the T2i on a 5D be pay careful attention the corners and edges of the image, especially at the widest angle setting. It's quite possible that the T2i lens casts a narrower circle on the sensor. The fact of the matter is with lenses, like so many other things, you get what you pay for and big glass costs big bucks.