If you plan on keeping your car NA then 2.5" pipe will do fine with out being obnoxiously loud. A good muffler is Magna-Flow, these are see through and have a good deep sound but are not too loud. For a P-type mkII supra you should definately get a muffler with dual outs, it just looks better I.M.O. Any good muffler shop can make this custom system for you for $250-$300 USD which should include a new larger cat., pipe, hangers, and muffler