What is the method to find the resonance frequency of a PowerFlow exhaust?
I think there is a misconception with how these systems work.  These are 4-to-1 or 6-to-1 header systems, as used on cars, and the primary feature of a header system is that the pipes between the exhaust ports and collector are exactly the same length, which is why, on cars, they snake this way and that, because the port closest to the collector has to have a pipe that is just as long as the port farthest away.

The objective is the make sure the exhaust pulse from each cylinder reaches the collector at an evenly spaced interval to minimize interference between the pulses, and also to have the low pressure behind each pulse help pull the next pressure pulse into the collector (they are sometimes called "extractor" exhaust systems because the pulse timing creates a kind of suction effect). 

The timing of each pressure pulse's arrival at the collector is the key factor.  To the extent that tuning goes on, it's to make tiny adjustments in pipe length to account for small variations in pulse timing due to different pipe contours - a pipe with more bends may need to be shortened a tiny amount to because its pulse arrives a bit late because of its more convoluted path.  In other words the tuning is the ensure the pulses are arriving at the collector evenly spaced.

From a performance standpoint, longer equal-length pipes are better than shorter equal-length pipes because the inertia of the flow stream is higher and this helps evacuate the cylinders better, enhancing the whole suction effect, kind of the inverse of the slight supercharging effect of long intake runners on the intake side.

The short stack Powerflow system gets 85-90% of the gain because it's still a 4-1 header, which is the primary feature; it's just that with shorter pipes there is a bit of efficiency loss from the shorter flow run compared to long pipes. You also save a couple pounds with less piping.