PLBROBST
I'm glad you were able to sort your engine. If it's like mine you wont have to touch the idle needle again.

I know of 2 guys running pitts style mufflers successfully on GMS .47s. Both had major initial problems and couldn't get the engines to run reliably. Eventually they both (independently) came up with the same 'fix'. The fix involved plugging one of the two outlet holes from the muffler. This obviously increases muffler pressure, so it seems that the standard pitts muffler doesn't produce as much pressure as the standard (or Tower) mufflers.
While I haven't seen either of these engines checked with a tach, I don't think the performance was as good as on the standard (or Tower) mufflers.