Bullet, you're an extreme case. Compare your mod list to his. He can't exhaust that much volume. You can. (I can't either).
The 3" pipe will provide more laminar flow by increasing the velocity. You can do the same thing with the 4" that you're running for several reasons. You have no bend in the pipe, so it'* more laminar, you are flowing more with your M90 and pulley, and you have the headers to get it out afterwards.
A 3.5" or 4" pipe will actually perform worse than a 3" on his car, but not yours.