This everyone should agree is the goal.

I've run an 8-6 on a TT 40 for break-in. Also as small as a 10-6 cut down to 6" diameter on a 40 engine. 40 sized engines are pretty much bullet proof well past 20,000 rpm.

I said I'b be surprised if it didn't reach 15K


There is a specific rpm that every engine produces maximum power. It is quite often listed in advertising or a spec page. Generally with just a muffler, it is a fairly broad range of rpm, say + or - 500 to a 1000 rpm from the stated peak power rpm.

You do not want to prop the airplane so that it turns this rpm on the ground. This is because every engine and prop combination "unloads" in the air. The amount of unload depends upon a number of factors, including the timing of the crank and sleeve, compression ratio, and the drag of the airframe. For most sport engines and planes, you can expect about 10 to 15% increase in rpm over the peak reading on the ground. This is one of the reasons you have to set the needle valve a bit on the rich side, so the engine does not go lean as it unloads.

So if the engine has it's peak power at 15,000 rpm, you would want to hit close to that in the air for maximum performance (generally either speed or climb). Working that back by 15%, you might select a prop that turns 12, 750 when peaked, but one that is from 12K to 13.5K would fall within a range that would work. Buy multiple props and test fly.

As stated by Jeff, not all props are equal or even equivalent when they have the same diameter and pitch. Variations of blade width, airfoils, and the pitch curve make it difficult to compare. Even little things like the ridge of the mold separation line can affect the prop and it's performance. The shape of the nose of the airplane may need a completely different pitch curve to work well. Test test test.

For most sport models I generally use just enough pitch to knife edge well, and trade away more pitch in the prop for more diameter to improve the climb.