fdiron, i doubt a p51 needs 400 mph to do a loop.
1st i am very astonished that the P51 gains alt during that, that would mean, it earns some E when looping. That seems to me a bid weird, but that would explaine, why a P51 can get an advantage when going over-the-top.
And about the speed, i think you can loop with the P51 once it is faster thatn 270 mph IAS, than it should work. It might not be the best looking and smoothest loop, but it will do it.