Did you try tuning the carb when the car went slower? Any other changes to the car? Having the mph and sixty foot times might help pin down the range that the loss occurred in. Based on this, I can see why you would say that your car went faster with a specific amount of back pressure in the exhaust system. However, I feel that more investigation and testing would likely show another less restrictive exhaust system and setting the engine up accordingly would result in a power gain - like your friend experienced.

Certainly one case isn't enough to condemn all Pontiacs to restrictive exhaust systems! Not to argue but you also said that Pontiacs need back pressure, specifically zero or minimal back pressure is not good for a Pontiac. That's what I disagreed with.

To reiterate, I'd advise dual 2.5" pipes with good flowing (non-restrictive) mufflers, an X-pipe and full length tailpipes. Pointing the ends of the tailpipes down and slightly in will also cancel some of the noise, allowing slightly better mufflers, which will make more power and better fuel economy. The main point is to design the system, not just grab the cheapest parts and stick them on.

I think that a well thought-out 2.5" dual exhaust would out-perform a poorly designed dual 3" system. It may not make the same peak horsepower but should have a better average output through the speed range. As with any pursuit of performance, the whole package must be considered and sized accordingly.