Sometimes, 3 inch will actually slow a car down due to the lack of back-pressure. I know that when I was running the stock size with no muffler, it was actually slower than stock. I put a Magnaflow on and picked up quite a bit.
I think that even with a header on the car, it should still sound good as long as you leave the resinator in line. That's what gives our cars that deep sound that the other imports don't have. No pissed off weed-eater sound with our cars.