1) RA2 doesn't give a damn about the display mesh - it could be a million-vertex piece of art, as long as your computer handles it you're fine.
2) Wheels are a different beast - there's not only a collision mesh (which does collisions), but also a traction mesh, which tells the game if the wheel should have any drag (based on the fact if it's colliding with any other mesh, like the floor). The traction could be a cuboid, it wouldn't really do any bad apart the fact the sometimes the wheel would drag more and sometimes less, since the radius changes a lot more ( (sqrt(2)-1)/2 * 100% to be precise) compared to a, let's say, 8-sided cylinder.
Also, I just reformatted my system HDD, so I won't be able to do any RA2-related stuff for a couple of days, until I get Windows installed (OS X only ATM, and I love it).
