Kind of, if you buy a decent kit the bags are unbelievably thick! It all depends on what sort of speeds etc you're doing and your set-up. I reckon if you're driving sensibly you'll probably need some arch-work and a new tyre.
Bag will lose pressure and car will lower itself. Usually sitting the arch ontop of the tyre. (4 way set up, you'll only lose pressure from 1 corner meaning only that corner will drop.. Rest of suspension will help keep car off the ground.)