If they're saying you can run 20's...goodluck, our cars arn't cadillacs lol. I mean 19s is fine but 20s is just over the line. I mean if you are stock suspension i dont see why not hehe. But if you are a crazy driver and would like to eventually auto x in the later future i'd just get a decent set of 18s, and nice set of coilovers, rogue engineering rsm's(rear strut mounts), and also make sure you have good control arm bushings which i just had changed because they tend to go out but if you change it before you lower your car you wont have nothing to worry about.
2001 BMW 330i