These are very good questions and I await comments from our resident experts.
To throw another component into the mix, I have heard some manufacturers claim their PWMs make better gas that others. And other claim the quality of gas decreases if you drive the cell to hard.
I have only recently started doing some "pop" tests, and I'm not really sure how relivant they are, at least to MPG gains. To keep things as "scientific" as I can, I got a small liquor bottle (like the kind they sell on airlines). It is small enough to be safe, I can hold the neck of the bottle in a pair of vice grips so my hand doesn't muffle the sound, and it small enough to fill quickly so i know it is always full.
I have noticed a difference in sharpness of the retort between a couple of systems, but have not been able yet to determine why. It's an ongoing test that I don't know will go anywhere.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.