Well, here is my take - right or wrong - We started with the factory Cats - the 105 Octane was burning them up - went to Stainless high flow and saw a performance gain then ditched them and tuned to clean from the E85 - Without the cats - we lost about a pound of boost because we lost needed back pressure. To offset that and gain it back we went from 2.5" to 3" into the muffer and then tuned it back to 2.25" for the tailpipe. We did this to get that pound back but today we blew out a zero restriction stainless muffler rated at 500 hp so we are replacing that next week with a different brand (not Walker). I suspect the dyno guy had a hand in this - LOL! With the elimination of the Cats we had significant gain in all departments but hindsight says that the boosted engine on reg fuel would have been just fine the way it was. Our changes rerouted the bottle neck in the factory pipe that is created for making backpressure to get us to the 446 RWHP numbers - good luck - if you increase your boost and dont see much gain - then I would say look at your exhaust performance.