It may not COST you power on a dyno, but in real life, in makes the car slower....wether it be cicuits or drag racing.

Also, the diff is unsprung weight.....which is even worse than body weight.....keeping unsprung weight down to a minimum is key. The extent that my mate has gone to, to reduce the weight of his diff in the sprinter is incredible, but it is significantly lighter than a standard sprinter diff.....and it also helped the car turn quick lap times. It also broke at the last race meeting , after 12 months of hard racing, and a fair amount of testing. He is now considering going to a full spool set-up - which is stronger, and also lighter I believe.