Just to close this one out. I was not confident the ST2500 would develop the grunt the Mustang needs and deserves and while pondering this a post pops up on fascistbook by a mate of mine who was selling a ST3000!! mmm, hmmmm me thinks, a tad longer and a tad higher but should fit nicely.
Deal done, swapped over the MMM petrol conversion bits to the ST3000, installed and set up throws.

Ran it yesterday on the Mejinik 17x10, 8350 RPM. WOW
Right I thought lets see how it likes the Bolly 18x10, 8000 RPM and engine is much happier as it like the weight of the Bolly. It actually produces higher static thrust on the Bolly than the Mejnik. Bolly 21.65lbs static on a 24lb model.
Flew the Mustang this morning, boy what a difference! It roared down the strip and into the air, had to pull it back to half as it was that fast in the air. Loops from level flight.

It did feel different in the air . . .

Lined up for downwind, drop gear, flaps etc and had to hold quite a lot of back stick, never had to do that before.
Came in with a bit of power on and pulled throttle back like I usually do to flare it on the mains and it was hard to flare then reared up, got it leveled and landed, bumpy but ok.

Hmmm me thinks. Checked CG with help of another chap and yes quite nose heavy, the ST3000 must be quite a lot heavier. Moved both flight batteries and the ignition battery as far rearward as I could which is about where the pilots feet are but still nose heavy. Had to put 1ounce right in the tail via the access hatch there. Just a very slight nose droop which should be ok.
Fingers crossed for next flights.

Very happy with the ST3000 though, what a beauty!