Well, what I meant by that was from the time I tried a Dynomax Turbo muffler (not the Super Turbo that I have now).
That was a very restrictive sucker, and the engine just felt choked throughout the entire rev range. Even the sound was snuffed out and it sounded like a little bee with a head cold. It was pretty awful.
This Super Turbo muffler is different in every way. My engine poops out at 5k too - that's from inherent cam profile, I believe - but the difference is it revs faster than it did and snarls its way clear up to that 5,000 RPM "wall" without any problem. The sound is really, really good.
Dave is probably right in that they're going to fall apart sooner or later, but so far mine's held together just fine for 30,000 miles. The good news is the muffler isn't an expensive one and can easily be replaced.