A 20" case Super Turbo has 3 x 14" or 42" of baffled tubing compared to about 22" in an Ultraflow, actual backpressure within the muffler could be measured by putting a pressure gauge through the shell of the case....I'm willing to bet that on most motors you wouldn't see very much if any pressure reading until a 440" motor got up over 5000 rpm or so, and even higher with an X or H pipe on it. On a 416" stroker smallblock in a B or E body I doubt you would see any ET difference if you used a true 3" Hemi reverse flow and very little ET loss, maybe .05 or less in a 0-60 run even on a 500" high compression motor.

The Muscle Motors test of a 3800 pound very low 11 second approx 550 hp car saw .14 seconds and 1.29 mph in the 1/4 going from a rusted out 7 yr old 20" Hemi turbo to a 14" ultraflow back to back, but the DB's went up (from 93 to 100)substantially. And that was on slicks, so with most cars and on street tires I doubt the real world difference would be as noticable as the seat-of-the pants meter might indicate.

Obviously a perforated straight pipe is going to outflow a reverse flow of the same pipe od....My only issue with them is anybody can buy them and for every well-tuned nice sounding Mopar there's a more than a hundred "hey look at me!" Boy-racers who really just make a lot of noise

These days I think I'm more impressed every day with the FAST guys doing it (reasonably quietly) with 2 1/2" duals and manifolds. The best sounding mufflers to me are the Corsa's but they don't seem to do well with big overlap motors. To me on the street I/ve always prefered a more stealthy low deep rumble that allows you to hear the 'chirp' of the high compression roller motor over what pretty much a generic loud 'open header' cackle type sound which just reminds me of walking around the pits for 40 yrs. I don't mean that as a insult to glasspack style mufflers but us guys that have been around a while can tell you they could tell the difference between a Ford from a Mopar or Pontiac just by the tone....from a half a block away....and that all goes away with a glasspack...so for me I guess it's more a nostalgia thing I guess...and maybe that's worth a tenth or so . But opinions vary, I guess.