I think that 2.5" is plenty for a 400 hp engine. It should be quieter, have less drone and package better than 3". Also the extra velocity and heat in the exhaust gas should help keep the exhaust from rusting out from condensation, without being too restrictive. And many mufflers that have 3" in/outlets actually are 2.5" cores with the tubes swedged out, and don't really flow any more.