I would have to go find some digging, but in the past when I tried to find weight, the 4 door GP was actually a couple of pounds heavier than the Regal. Most of the time when people have shown me a lighter weight for the GP it was in the 2 door version. As far as "which car is faster", I only raced a couple of grand prix, but they were literally neck and neck. The only difference was in the driver and not in the performance of the car.