You will get a fair amount of lift just using adjustment on rear trailing arms (about an inch or so). But it depends where they are currently set. If they have been set low, you will get an inch or so of lift. If they are already high and not enough, you will need to re -set splines on torsion bars. But you don't want to go much higher than factory limits just to clear your driveway - the handling will deteriorate.
http://forums.pelicanparts.com/porsche-911-technical-forum/374522-torsion-bar-problem-maybe-something-else-post3615282.html?highlight=Alan#post3615282
Alan