Took these three with my 10-22mm for such a wide angle on a crop body its distortion is very well controlled you just have to make sure you set yourself up correctly, generally get down on one knee instead of standing and lowering the camera. (just basic stuff)
Saying that the second image you can see the edge of the wheel pulling. Really for car photography you need to be using over 35mm because that will give no distortion at all anything under and the car will pull in one respect, but when shooting on a rig you cant do anything else. I prefer to shoot with my 70-200mm L because its an incredible lens although you have to be careful with that too because it compresses perspective (bringing the background closer to the foreground).
Blurring at the edge shouldn't really be happening, this lens isnt renowned for being soft at all, even at F4 it is sharp but stopping it down to F8 will combat anything over F11 the lens wont be any sharper like the sweet sot. If it is soft your copy may not be in sync with the camera. Canon build the lenses and body within a tolerance of 3 so +3 to -3 but if you get odd copies of each say a -3 lens and a +3 camera, areas of the lens will not be sharp. The best way of combatting it is using the micro adjust feature (but I dont think the 550D has it just the 50D and the 7D in the APC range) or send it away to be calibrated, or taking it back and getting another copy. This is probably the reason for the difference between your 550D and your friends 7D.
There is no difference in the type of sensor in the 7D vs the 550D they are both APC sized, but just different generations and resolutions, meaning there wont be any change in distortion. EF-s was specifically designed for the crop APC size and they did everything possible so the lenses wouldnt be interchangeable with there full frame EF only cameras like the 5D/1D. Infact the sensor in the 7D has too many pixels 18mp is too much for APC and the biggest complaint about the newer 18mp sensor in the 60D 600D and 7D is that it is extremely noisy at ISO100+200 similar to the previous generation cameras (40D&50D) at ISO 250. But it is a fantastic camera
But if you are wanting to replace the extremely poor kit lens then the two best ones for EF-s are the 15-85mm F3.5-5.6 IS but this is a variable aperture lens or the 17-55mm IS F2.8 which I also have and is by far the best lens in the EF-s range pretty much an L series in quality without the weather sealing. Bu the 15-85mm is a cheaper wider and longer and is a great addition with the IS but 5.6 at 85mm is pretty slow, I shoot a lot of night photography so the F2.8 all the way through the range is perfect. The EF lenses 24-70 or 24-105 are also great but not on a crop camera because they are equivalent to 38.4mm at the widest (24x1.6) because the APC is smaller you have to multiply the focal length by 1.6 to get an accurate focal length. Works great with longer lenses. My 70-200 is the equivalent to 112-320mm at 2.8 add a 2x extender to it and it becomes a 224-640mm F5.6 lens!!! amazing for birding or any sports photography just not as sharp but its a trade off a 600mm lens is over £6k. Many people use crop cameras to get the extra length out of the lenses.
Im getting myself a 5D MKIII :evil: I have a 40D atm because I havent found anything worthy of replacement except the 5D MKII so will use both with the 40D as a backup
Then the £2000 for the 24-70mm F2.8 II 