Adding to the fray here. Perhaps instead of looking at just the exhaust timing, whether high or low, what about it's relationship to the intake timing. The blowdown timing (when both intake and exhaust ports are open) would have a huge effect on the pipes ability or lack thereof, to do it's work. Too high and you can have a peaky somewhat torqueless motor, too low and you can have an easily driveable motor that won't have very high peak power. Dave Marles for one, stresses the importance of this when looking to modify a motors timing. After all, the entire system has to work together. Just a thought.
Actually, it is much more complicated than that. BUT, you are correct in the analysis of "BLow Down TIme" being important for a pipe to operate efficiently.
It gets into time/angle/area of each of the ports AND the intake tract including the carb. For an engine to operate efficiently, there should be a balance in each component. If one is restrictive, it will limit the output of the engine.
The statement that "After all, the entire system has to work together." is correct, but involves the time/angle/areas of each component of the porting system of the engine.
Marty Davis