On my sons Dart he runs the Flow Masters and when all capped up it slows the car almost 2 tenths. Just space the header collectors with a few washers and it picked up the almost 2 tenths. Its a little loud but who cares as its a hotrod.
I run the Dyno Max Ultra Flows with the full 3" exh out to the rear of the car and I really like it. Its not to loud unless you step on it pretty good. I have never uncapped my car to see if my exh slows it down any. Ron