My Saito 72 turns an APC 13x7 at 9850 rpm, my OS 61 FX turns the same prop at 9950 rpm. Seeing as the 61 weighs nearly 1/2 lb more when you include the muffler, it is not worth it in terms of power.

Now using a 13x7 on a 61 FX is over propping, but necessary to keep the noise down (we have strict noise limits in the UK). So seeing as I have to use a big prop at lower rpm anyway, I now only buy four strokes as they are designed for it.