Well, the Xoar props are just the best IMO. Highly precise, efficient, and come pre-balanced. Worth every penny IMO. But a less aggressive prop.... Believe me, they are not considering the aircraft. This airplane had LOADS of drag. You will want the 12"pitch to get out of trouble spots. Halving the pitch will increase your thrust, yes, but you will lose airspeed... a LOT of airspeed. You will be struggling to keep it in the air. I tried a 16x10 and it was marginal. The 18x12 will suck up some power to overcome the drag but that is expected. You have a rather large frontal area, a HUGE scoop on the bottom, two wings, and struts. Plenty of drag... even without the elastic cords. While a 18x8 would give you good thrust to overcome things, you would still be struggling with airspeed... which is what you need to keep flying.
Math lesson:
32Kv motor * 25.2V = 8064rpm
8064rpm * 8 inch prop pitch = 64512 in/min = 61mph
8064rpm * 12 inch prop pitch = 69768 in/min = 66mph
so, you can quibble over 5mph all you want but if you are hauling around a 14-pound airplane you better be darn sure you have the airspeed to do what you want to do.