Originally Posted by Crash

^^^ +1 Exactly!

The car doesn't have enough torque as it is... The motor is likely already flowing exhaust as best it can as is... The same amount of air is going to flow out 3" as it would flow out a 2.5". And with 3" you lose torque because you're losing back-pressure. The only time you need to open up the exhaust is when you have MORE air coming in than can flow out at equal volume.