Header sizing and length is where your power is.
All you want the exhaust system to do is get the gases out with minumum backpressure
2.5 pipe is good for 450hp (in theory). mufflers will make a difference too. Use a mandral bent system with a straight through muffler design such as the Dyno Max Ultra flow. Mandral kits are no more money that a muffler shop kink job
real world muffler testing
http://www.boyleworks.com/ta400/psp/exhaust.html