My OS 61 FX does 10200 with an APC 12x10 with pipe. 9200 sounds about right to me, Super Tigre's are powerful engines. I could get my 61 FX up to 10800 with 12x10 but I richened it down to 10.200 so it would'nt lean out on takeoff.

Never had an engine run so much differently in bench and in the plane but it sounds overly sensitive to me. I'm surprised lengthening the pipe doesn't help, it should be easier to tune when it's longer. I've no experience with Macs pipes but try a Hatori. I have a purple pipe which is three chambers like Hatori and it's superb. Good throttling and easy to tune. No fuzz at all.