The exhaust in the pipes isn't consistently "full". The exhaust is in pulses from each cylinder - "waves". So there are actual low and high pressure areas, but all moving pretty fast. The trick is to optimize the lengths and diameters of the headers, including and X or H additions in the system. Hold your hand to an exhaust pipe and you'll feel the pulses.
I put a 3" X-pipe and UltraFlo mufflers on my race car. It picked up between .05 and .10 over an open exhaust with collectors. I believe carb liked 1-2 numbers less jet as well after jetting was as good as I could get it with open headers. X pipe helped and those mufflers didn't hurt. I would think that 500+ HP would benefit more from a 3.5" exhaust.
