That's interesting...
Plugging those numbers into the typical drag race calculator:
http://www.eldemoniorojo.com/calculator.htm
With muffler: 265 HP @ wheel
Without muffler (assuming ~50 lbs weight removal): 268 @ wheel
Well within statistical error range. In fact, the car weighing less sans muffler probably, by OP's account, the conditions on the launch with the muffler was ever so slightly WORSE. I guess we can safely say that the presence of the muffler makes very little statistical change in HP output (at max HP).
Which is safe to say that any muffler only exhaust making any real appreciable HP are pretty much false claims.