Have you actually used the calculator linked previously to plug in a 15 x 8 at 9300 RPM?

15 x 8 @ 9300 RPM 68*F generates 11.94# of static thrust and absorbs 2.213 HP

You think that might be a typo on Horizon Hobby's site?

So maybe they meant 16 x 8.

16 x 8 @ 9300 RPM 68*F generates 15.54# of static thrust and absorbs 2.865 HP

Sound more reasonable?

Lets plug in an 18 x 8 @ 7900 RPM as I have observed with my FA-180 with glow ignition.

18 X 8 @ 7900 RPM generates 17.86# of static thrust and absorbs 2.813 HP

Hmmm, 15% more static thrust and more RPM in reserve for higher air speed. Seems the 18 x 8 is more efficient.

So now lets plug in the RPM numbers observed for my CDI stock FA-180 and the various high compression versions with CDI.

Otherwise stock FA-180 with CDI
18 x 8 @ 8000 RPM generates 18.32# of static thrust and absorbs 2.921 HP

FA-180/FG-57 piston 11:1 CR with CDI;
18 x 8 @ 8150 RPM generates 19.01# of static thrust and absorbs 3.088HP

FA-180 .032" deck reduction 12.8:1 CR CDI;
18 x 8 @ 8300 RPM generates 19.72# of static thrust and absorbs 3.262 HP

Same engine with 12mm FA-220 big bore carburetor and ported intake manifold with CDI;
18 x 8 @ 8450 RPM generates 20.44# of static thrust and absorbs 3.442 HP

If I up the nitro from 15% to 30% and reduce the lube content to 8% the same engine will turn the 18 X 8 @ 8850 RPM
18 x 8 @ 8850 RPM generates 22.452# of static thrust and absorbs 3.954 HP