I think that .91 four-strokes should be swinging a bigger prop at lower rpm anyway. A four-stroke turning above 10k seems a bit much. A 15x6 gets it in the 8k range which should be more in the power-band on a four-stroke since they make their power more with torque than high rpm.
Modern four strokes make more power at hight RPM than they used to. Many now make their best power above 10K My old OS .91 did best just under 10K my TT was happier in the low 10's. As I recall the OS .91 would turn a 14-6 at about 9,800 and the TT would turn it at about 10,200 or so. This was on 10 or 15% nitro. With a larger prop like a 14-8 or 15-6 they were almost the same, but I never compared them side to side with a larger prop. Your manual will tell you the maximum RPM, stay well below that and you will be OK.