Talk to most performance muffler shops and they will tell you that a dual 2.25 system is good for 400HP, beyond that you need to go up to 2.5".
This link has a chart which is based on maximum flow that says 371HP for a 2.25 system, more than just maximum flow tho you need to be thinking velocity as it leaves the engine so a 3" system may be too big, you will need to calculate the estimated HP and then make a decision. Google "exhaust pipe flow rates" and have a good read.
http://www.exhaustvideos.com/faq/how...pipe-diameter/