Dead on moonduck. Good to see you are applying yourself !
About the backpressure/scavenging effects : My theory on that is , you need to close your exhaust valves for maximum compression. With a supercharged wide-open intake ( such as a dragster ), you already have more compression than can ever need and more pressure going out than can be dealt with, so the loss by low backpressure is much less than the performance gain by blowing flaming nitrous out...
Another thing: You see individual runner open headers only on drag racers. F1 cars have 8-2 systems or 8-4... I have never seen them running 8-8... Of course, I could be wrong, but I think that's pretty true. Nascar runs 8-2. WRC? 4-1
About the sucking noise : I'm willing to guess the car is being run without an airfilter, and as the secondary barrels of the 4 barrel carb kicks in, it creates the sucking sound from the immense amount of vaccuum caused by the engine as the engine draws huge CFM through the 2.5 inch hole of the aircleaner. I had an old pontiac which did that.
If you want to read the importance of scavenging, read something on two-stroke expansion chambers. two-strokes wouldn't run for shit without them