2010 Ford Mustang Speed difference question

Not so much a a repair problem but a buddy of mine has a 2011 Mercedes Benz e550 Naturally Aspirated and its all stock except for a small tune. The sheet says it runs stock 382hp and about torque. I have a 2010 mustang gt with a 5.0 v block swap and an offroad xpipe to muffler delete system. He weighs around 5500 which is a little over 2000lbs more than me. I was wondering what could possibly be affecting his car internally so much as to allow him to beat my cars quarter time by about 1.3 to 1.5 seconds? Thanks.