i'm assuming you mean a 1991 318is, so no there would not be much of a difference. i'll quote this from bimmerworld magazine:
"the 318is also got sport suspension, a front air dam and rear spoiler. Factory options were confined to a limited-slip differential, a manual sunroof, heated seats (convertible only), and fog lights. Both cloth and leatherette upholstery were available on all models."
ok so what engine swap do you want to do? the m20 (either the 2.5 or the 2.7 eta engine)? because it uses different motor mounts than the stock engine in the 318is (m42 engine) and having an m20 engine swapped into it would be very difficult. the only cool thing about the 318 is that it has the 4.1:1 differential in the back with optional limited slip but it would be way easier to get a 325i or 325is and just swap the differential (the 325i/is use a 3.73:1 differential). trust me, it will be way more costly to swap a different engine into the 318.
if you are concerned about performance just get a 325i/is, upgrade the suspension (new shocks, springs, maybe sway bars) and then get a chip, K&N cone filter, and cat back with the 318 differential. if thats not enough for you they do offer headers and even a more aggresive cam.
