Actually, the '89-'93 Devilles are up to 500lbs lighter than their '00-'05 counterparts. In fact, the '91 Deville was even lighter than the '91 Corvette ZR1. After the switch to the FWD platform in 1985, the "Old Boat" Cadillac was no more. The older cars have less top speed, simply because they have a higher co-effcient of drag, and less horsepower to work against that drag (a '90 Deville only had 180bhp, a '00 had 275). But they're not really that much slower, I know a 4.9L powered Deville has no trouble hitting its 112mph limiter.