I've met comparatively few people (outside these forums of course...how's that George? )that seem to have more than a passing understanding of exhaust backpressure and how it relates to a 4 cycle exhaust system. If a conventional muffler (turbo or otherwise) is trully "restrictive" It is logical that you would be able to measure positive pressure against the walls of the muffler case because the gasses can't get out as fast as they are coming in. We've probably all seen 'blown out' mufflers or cases that have been swollen (if only slightly) by excessive exhaust pressure.

Also, more surface area (which includes the pipe walls themselves and absorbtion is evident because the pipes get hot) there is in a system can actually aid power because by absorbing the exhuast heat you are cooling and slowing the energy that remains in the system until it exhusts to the atmosphere. EX On a Big B or E body with a 3" system there's a LOT of pipe upstream of the mufflers that aids in reducing the remaining energy before it ever gets to the case. Also, the net effect is really at what point does the exhuast slow the rate of acceleration of the motor? Assuming no tire slip, a given car/motor going from idle to
WOT with 3.55 gears vs say 4.30 gears, a more restrictive muffler will have less of an effect on acceleration because the peak volume of air moving through the engine meters through the system CFM per second at a lower rate.

Some People erroneously assume that flowmasters are 'restrictive' because they flow less CFM in a side by side 'free flow test'; this really has little if any bearing on what an actual 4 cycle exhaust system does), a Flowmaster (and similar designs) operate very much like a check valve in that they are designed (in theory) to capture and channel the FORWARD moving" exhaust pulses while reducing the reversion effects of the vaccuum created behind each exhuast pulse as the valve is closing behind a high velocity stream. In an open pipe atmospheric air naturally fills the void and as we know air has mass that must be 'pushed' through the pipe before the exhaust exits the system.

I don't mean this as a 'dissertation' at all, I'm just pointing out there's a lot of HYPE and "seat of the pants" testimonials that don't always stand-up 'real world' to sound engineering logic ....and timeslips. On a race car, sure but on the street...

opinions vary of course