Great comparisons and data.
Couple of uncertain items:
The decibel meter brand used was not mentioned. Not all decibel meters have a flat response(treat all frequencies equally).
Depending on the frequency of the exhaust tone, the meter can give better readings for some freq.than others if it attenuates a certain frequency band that is outputted by the exhaust.
If would of been nice if they could of also measured the frequency of the measured tones at the measured RPM's to see which one had the lowest tone and which one had the highest tone and at which RPM.