I've got a dual 2-1/2" inlet, 3" outlet Flowmaster on one of the gas trucks and it's fairly assertive as far as sound. Also got Thorley headers on that thing. A 360 gas Dodge. Did careful measurements with a GTech Pro before and after the headers/muffler (it had a stock 318 with catalyst before, VERY restrictive looking) and the net increase in performance was astounding. In the negative sense. Sounds better, feels better, runs EXACTLY the same.
But turbodiesels are different animals and respond very well to any decrease in backpressure without exception.