Thanks for writing...
The VQ P-51 I have was built around 2001. My understanding is that they improved/changed a few things on the warbird and on the Dago versions.
But a big issue I ran into was that the stabilizer (built up stick structure) was WAY underbuilt. If pushed to a high speed, the entire horizontal stabilizer would flutter.
So I can not really tell you how fast it can go. I had to slow mine down, or it was going to self-destruct.
With my plane, I have the SJ-46, turbojett, and I run either an 11x5 or 12x4 prop. The 12x4 prop slows the plane down a bunch, but allows the engine to unload a bit. Plus, with the 3" spinner, the bigger prop worked much better for take off and landing performance. Very scale like flight down around 1/2 to 2/3 throttle. I only take it to full power on vertical uplines or if doing aerobatics and snaps.
I had flown the P-51 with a 10x6 and it really moved along and then buzzed the stab. So again, it probably can be fast, but I never got it that far.
Using the big props on the SJ-46 and turbo was an experiment to push the muffler to its limits. The combination turns just at 14,000 rpm peak, and I fly it set down around 13,200-13,400 for launch. Once in flight, most guys think I have a .61 engine in the nose
It unloads just a bit, and it has no trouble with the big prop. But the 12x4 is as big as you would want to go.
With the OS55AX you can run the 11x6 or 12x4 without any problem. An 11x7 and 12x5 are worth trying too. But let the engine unload a bit, look for that 13,000-14,000 rpm range at ground peak. Aircraft will probably perform VERY well.