Juice,

You are doing it right.
To get more speed, you must get more RPM and use a higher pitch prop.

But drag is also an issue. The P-51 is one of the fastest planes of WWII, but you WM still makes some drag, especially because despite its scale structure, it does not have a laminar flow wing, like the real thing.

To get more speed out of your model, you can reduce the drag by flush fitting gear doors and smoothening the control surface hinging surfaces.

But you need a lot more power. Your OS.46FX is not the power paragon you apparently think it is.

You should get a Jett .50, or another very strong engine.
Yes it could cost you. It will get your P-51 to high speed, due to high RPM on a 10x6.

If you go for a Jett .60, or a YS .61 two-stroke, you can get the same high RPM on a 10x8, or a 10x9, which can bring you to a 30% higher speed.

It doesn't matter how much nitro you use, a common .60 engine, will not bring you to the RPM you need, on a usable prop, for the speed you want. Neither will your .46FX.