Depending on the dyno, as you approach the 500whp mark you should definately consider an upgrade from the 2.5 inch exhausts. We've done many 2.75 inch exhausts in the 350Z world up past 650whp without any restrictions. Although 500whp is achievable with a 2.5 inch exhaust, it's not something I would recommend. The heat load on the engine will be significantly reduced if you run the larger exhaust, so you will see more power safely.
