it depends. Sometimes it does increase power, SOMETIMES. I know the stock EVO's pipes are sh*t and upgrades to those are beneficial.

With the larger pipes you are going to have better flow, common sense. If you plan on going on higher boost, larger pipes are always a plus. If you keep stock boost and get the larger pipes, you might have lag..MIGHT. It would be minimal if you did.

I had 3" piping on 15psi. Some people still run 2.5 on that much boost and more. Just depends on what you like.

-Ant.