there was a special on the speed channel for a bag companie where they had two camaros with and with out bags run a road coarse and the one with bags did run faster, but there is a difference with the systems between a proformance bag setup (like the Marks have) and the lowrider style bags.
one of the guys i work with has a pretty sweet air ride system in his S10 and when we were checking out all of the options for his truck, the guy at Chop and Drop Custom Shop, showed us all the different types of bag systems he could go with. some where for slaming and layin frame, and some where for sport use. he was telling us that if he went for the sport setup, it would wear the bags out early to run them low all the time. since he wanted to be able to park it on the frame, he went with a bag setup that was fine if it was driven around damn near empty, and there was a ton more body roll than when he just had the lowering kit on it no matter what height he crused at.