Quick rundown:

2003 IS300
Powerdyne Supercharger @ 7psi
aftermarket 6-2 header (removed 2 catalytic converters)
stock catalytic converter in 2-1 section of the header (I have a race pipe for this as well)

As the car sits right now, the low-end and mid-range power and torque are very good, but the power is tapering on the high end. Since the intake side is done, I'm looking to do the exhaust.

On that note, what size would be most beneficial for helping the high end? I'm thinking a 3in with a good-sized (24in?) resonator and a 3in Vibrant muffler will be good. I don't want it to be rediculously loud, but I want it to flow well. Will the piping be too big for the engine and modifications on the car?

Thanks for the help

 

You will see most people running at least a 3" exhaust on forced induction applications.