If every cylinder has the exact same compression as you stated I wouldn't care if a raccoon could be sucked up the exhaust pipe. Do yourself a favor and do a search on "exhaust reversion". They sell anti reversion valves for around $50 if you insist on keeping your dollar bills wrinkle free. We did a lot of research on exhaust flow since it is free horsepower and legal. While you're wasting time on reversion go ahead and check out "scavaging" and "Coenda affect". I don't think you have a problem but first hold your dollar bill with the engine off idle as ANY engine has exhaust pulses if turning slow enough and second have it scoped as a good scope man can spot a valve or guide problem immediately. Does a vacuum gauge give a steady reading? Yes, vacuum gauges can also be used to detect an exhaust problem. Don't be in a hurry to pull the head.

Evan