is there a hp diffrence with tailpipes vs. turndowns

[Ron1]

Race engines only? No. If you read the quote from David Vizard, he said all 4 cycle engines. I think what you are talking about is what he calls resonant tuning (not backpressure). That occurs in the headers, when it creates a low pressure pulse. Quote.." This reflected wave-flow initiates air-fuel flow into the cylinder through the open intake valve before the piston begins the intake stroke."
You may want to do some reading on the subject by picking up his books. He has been doing this since 1968 and has written 100's of books engine performance including extensive testing on exhaust systems.

Ron