I don't remember running a prop smaller than a 7x6 prop on my Fox .15's. Usually it is a 8x4 prop most of the time or a 8x3 for combat. I have a Fox .15BB engine on my mini-Sportster plane, and it is running too good with a 7x6 prop on that plane. So i wind up flying it around 1/2 throttle or so. For combat, If I remember right, I had to richen up the engine a little so it didn't rev higher than the max RPMs they allowed at the time using a 8x3 prop. But it has been a while since I flew RC combat though. I was getting around 14,000 plus RPMs using a Fox .15 BBRC engine.
I test ran a Magnum 15XL engine a while back with a 8x4 prop, sans muffler too, and it was running about equal and maybe arguably slighty better than the Fox .15BBRC engine. But not much difference at the time. I haven't tried a Magnum .15 XLS engine yet.
I seemed to have misplaced most of my Fox .15 videos, except for this one of a Fox .15 bushing engine using a 8x4 prop. The Glo-Bee tachometer didn't like working in the shade too much, but it looks like it was turning around 12,300 RPMs sans muffler with the 8x4 prop on it and 5% nitro-glow fuel. The muffler wouldn't fit on the engine while on the test stand. What was surprising was how well the engine idled like that without a muffler.