My second hand knowledge of the Fora .15 is it runs harder than most others because the crank case has been "stuffed" to increase pumping efficiency. The piston skirt actually passes into the backplate. I do not know anything about the different versions there might be of this engine, or if there is just 1 design for combat, free flight and control line.
About the Axial .28 turning a 7x10 prop 18,000 with no pipe.....it's able to accomplish this impressive feat because it is not a pipe timed engine, [I'll bet]....?.
I'll bet it's set about 160 degrees if not less....?
I would like to see what a Nelson .36 [low timed] could turn that 7 x 10.