it will save weight yes but it will create more under body drag...

the stock muffler / nismo muffler act as under body diffusers... if you remove them and put a single canister muffler / injen exhaust / greddy evoII they leave a big gap infront of the rear bumper... this will act as a big air catch and slow the car down...

3 stock cars weighting the same... one with NISMO exhaust one with INEJN exhaust one with Single Exit Canister exhaust...

the Single Exit Canister car will slow down fastest from the extra drag,

 

What jason is saying is right. But i never really noticed the drag with either of my exhaust setups, but I'm sure if you were to put the cars in a wind tunnel, you'd see teh differences. That makes a lot of sense, the stock exhaust has that panel that bolts to it to help guide the air.
Good point....

 

Unless you buy the APS dual catback exhaust system.



"Further good news for the performance enthusiast is that the APS High Output Exhaust has provision for the stock wind deflector on the leading side of the muffler assembly. The stock wind deflector is an aerodynamic aid that slices through the turbulent under body air flow and channels air under the muffler."

 

Also you could buy Titanium Brakes, which wiegh something like 4.5lbs each. Thats big rotational wieght savings at a big price of course.

 

for a billion dollars you can buy Amuse dry carbon body parts....

 

Too bad that kit is fugly