I didn't think there would be any bad tendencies if highly powered. Certainly the P51 could go for some more speed, but you reach a point where the drag will begin to creep up on you. I know switching to AA NiMH cells in mie added enough weight ofer the AAA NiMH cell that it handles entirely differently now (faster landing speeds, a bit less sensitive to control inputs). Just some things to consider. I like to have 'scale' like speeds so I tend to shoot for a slower than average speed in any scale type model.
An example: I have a 'similar' sized Yak which is all glass and will fly well on an AF 010 and 10 cells direct with an APC 5x5 as the airframe is pretty slick. I would not expect similar performance with teh same setup in the GWS P-51 as it is more draggy - at least from appearances.
Hey, I am for whatever works, but I am also lazy
So I tend to go for what is easy, and it does not get much easier than the stock gearbox with a brushless motor installed. Shoot, I have a Hacker B20-15L I am contemplating installing in a GWS P-51 for kicks and grins. I am sure there are many combinations that work better than the stock setup. It's all in how you want to fly.
The weight of the GWS P-51s seem to fluctuate between 15-20+ ounces depending on equipment, tolerances, etc.