Although you were only turning 9500 on the ground this engine prop combo will unload quite dramatically in the air. 12000 plus will be attained in a shallow dive.

It is also worth remembering that high compression 4st engines like Saitos give a prop a hard time esp at low rpm full throttle spool ups.

If you were flying 3D stuff with lots of throttle action this might be a contributory factor.

P.S. I am off to have a VERY close look at the APC 16x4W fitted to my OS 120 AX.