I believe I've seen the same data as you - there isn't alot out there; Without getting bogged down all I'll say is that I believe any decent silencer (Lexx, LeoVince, Q4,etc.) offers a bit of an increase in the higher ranges without any negative effect in the lower rpm ranges.Don't want to generalize, but . . . independent dyno charts I have seen show higher horsepower at some regimes with the OEM muffler than with aftermarket silencers. At other rpm regimes, the "trick" muffler registers modest power gains over the stock exhaust.