3 inch?
Throw that in the bin and put something decent on there.
How much boost and what fuel are you running to achieve those figures?
I'm running a twin 3 inch system on my car.
As a comparison, GT RS's at 2 bar made 414 rwkw using a kakimoto full mega N1 (rubbish catback which was 80mm into a 90mm into some peanut rear muffler which at best would have been 2.75 inch inside). Going back to -5's, from pods to stock airbox and the much bigger exhaust netted 426 rwkw at 23 psi, cam gears and the rear of the setup remained unchanged. Same car, same tuner, same fuel, same dyno. At that power level there is much to be gained.