P-51 high speed handling problems
I own a few examples the Model-Tech .25 Mustang ARC.
Very strong plane box stock. I had a 32SX in one with retracts , radared at nearly 100mph. The wing is not a problem on this model. I have another with a Nelson QM40 engine.
Your CG on the spar is right.
Are you hunting for altitude? Lots of down trim?
Any rudder trim?
The one common ailment this model seems to have with power and speed is known to be associated with the stab incidence. If I recall its rather high, should be around 0 degrees for best performance at speed.