A strictly vertical fight will be very even, at lower altitudes, as the Mossie has the same climb rate/acceleration as the P-51D. I've never had any trouble staying on top of a P-51 when I start with the E advantage. P-51 won't be roping a Mossie VI unless it started with a lot more E, and due to the Mossie's center mounted cannons that could be a dodgy tactic. A sustained spiral climb will favor the P-51, but if you've gotten into a situation where you're using that tactic you've already failed at the aggressive BnZing tactic. Players being even, a fight between a Mossie VI and P-51D should go to the one that started with more E. Just hammer the E out of the guy with less E and then kill him. Neither plane builds E fast enough to avoid that when being forced to bleed E in order to dodge.
In my turning comparison I was speaking in the horizontal which a lot of P-51D drivers will do against the Mossie and, as I always found it made for an easy kill, I'd accept. Only a few fighters I'll willingly turn fight in the Mossie: Me410, P-51D, Tempest, Me262, Fw190s and P-47s.