This might have been mentioned already but (I just breezed through this post). JCWhitney had exhaust tips in their catalog a few years back. The tips had a butterfly valve in them that opened under a pre-determined amount of back pressure. They would open around 2500 rpm I believe. I might have even been adjustable. They were nice stainless steel tip for about $60 bucks a piece but I haven't seen them since. They might still have em. It would be worth while to give them a ring maybe.