13.5K was about what I was getting with stock muffler, baffle removed. With Ultrathrust muffler (yeah, I know, $59) I get 14.7~14.9K. I don't know anything about the GMS 47, but 13.5K with this prop ain't bad....

Rest assured that the Irvine 53 with 12.25 X 3.75 will 3D a 5 lb plane with either muffler..with the UT my 5LB SE hovers at quite a bit less than half throttle..it's not even getting any gain from the UT muffler at this throttle setting.

As for understanding..I would compare both engines with the same prop..I'm not sure what the GMS 47 would turn with an 11 X 7, or what brand you tried.

A guy was flying a Pizzaz w/OS 46FX and 12.25 X 3.75 APC, then flew my SE, and was blown away by the power difference, same prop. He started looking for a Irvine 53 the next day.

I'm not trying to talk you out of the GMS 47, but I can say that the Irvine 53 is a very nice engine with the 12.25 X 3.75 APC.