RE: Yet another DDUST take II
the speed of the prop doesn't take into effect the prop's lift, only its pitch. Its actually (theoretically) possible to exceed the pitch speed of a prop for a given rpm - due to the lift the airfoil of the prop makes.
Keep in mind that when a plane unloads in the air, 10% or more rpm gains aren't unusual.
According to thrusthp (which isn't the most accurate thing there ever was, i'll grant you that) at 16,400 with an apc 8x8 prop your producing 1.032 hp. With an 8x7 apc, you would spin 17150 rpm to produce the same hp. Couple that with 10% unloading in the air, and your at 19000 rpm - which gives you a pitch speed of 126 mph.
If the 8x8 isn't unloading in the air, and the 8x7 does, your looking at it being worthwhile to try. But, when it comes right down to it, if you really want to know, the only way for sure is to try it with both, and either time it, shoot it with a radar gun, or doppler effect - and see which one is actually faster.