That doesn't make sense to me.
From my own personal experience, when I uncap my exhaust (2.5" Ultraflos with H-pipe, long tube headers), I pick up both low and top end power. Not noticable, but shows up as .15 on my ETs.
The reason why you sometimes get an increase of torque with some backpressure is because the exhaust velocity is higher, which raises torque output. At some point the high exhaust velocity builds enough momentum to "draw" the intake charge in better during the overlap period.
The reason why the top end falls off with an exhaust system is because typically the system cannot flow enough exhaust gas, which is needed because there isn't much time for the exhaust gases to be expelled out of the cylinder. Because of the restriction, the new intake mixture is contaminated with burnt gases, and thus produces less power. There is also a pumping loss, but not as significant. There are mufflers out there that actually can produce more HP than an open pipe, but you'll find that the backpressure in these exhaust systems are also darn near zero.
It seems like from your post that you say that the Mustang lost top end HP due to LACK of backpressure...which can' t be. You'll see that most to all dragsters run near zero exhaust pressure, and sometimes the scavenging effects brings the pressure into the negative digits. You only NEED backpressure if you for some reason had a poor combination of parts that result in low exhaust velocity.
The concept of the chambered muffler is not to flow a mass amount of air. The concept relies on sound/pulse wave cancellation & destruction, it has nothing to do with being naturally aspirated. The only reason why the muffler is effective power-wise is because Flowmaster has put a ton into R&D of these mufflers. There have been many back to back tests (Dragstrip tests) to show that while Flowmasters aren't the best muffler, they certainly aren't far behind from straight through mufflers.
I don't know much about turbo'd cars, so I can't debate too much. But what I do know is that because chambered mufflers rely heavily on exhaust pulses to do the job, they aren't the best choice for a turbo'd car. The turbo modifies those exhaust pulses as the exhaust exits the turbo, and chambered mufflers weren't designed to deal with that kind of gas flow. There is much more to this, but this is all I care to write about.
