Well I'll confirm you this.....
hopefully get around to doing a full post on this soon, but that rear **** is restrictive as hell, drop it, or change it, do something just don't keep it there if you want power.
pulled 421 stock, added high flow cats and 98+ headers, leaving the OEM rear **** in place. New pull 418, not good.
my theory other than my car is a piece, is the increased flow is hitting the OEM **** and shooting even more airflow back towards the engine, hurting power moreso than stock. Hence my assumption that it is restrictive, I'll dissect mine and take some pics when I drop that thing, sooner the better.
my theory was to isolate the gain of rear **** delete one step at a time, this confirms that airflow gains (intake, exhaust) is not something to be toyed with one step of the chain at a time, much akin to the funnel analogy.
now if my 418 will jump to something more respectable when the **** goes, like say 20 or more is the question.....
Might have to concede my Blue car is the most power banter if something doesn't happen here!