All I know is that a well flown P51D vs my Fw190D9 in a 1 on 1 fight is one of the best experiences I can get in this simulator. Have had the luck to find a couple of such fights last week, and enjoyed them immensely (one was a draw as I had to dive away because another enemy was getting dangerously close, the other I won with a very very complicated high deflection shot I was lucky to hit with as I'm a terribly lousy shot).
That the P51D is a very good matchup for the D9 is a big hint of how good the Pony is in the MA. As for how to defend against a faster plane on your six, it just requires a skilled pilot who knows how to get the best out from it.
Usually that means that for starters the pilot doesn't let his SA slip so low as for finding himself in such a situation, BTW. If someone faster than you is stuck like glue on your six, you did something wrong. And the plane has nothing to do with it.
just my 0.02$
The P-51 can turn VERY well when flown properly at the altitude and speed it was designed for. It can out turn anything it cant out run given a realistic matchup (Allied vs Axis). The 109-K4 might be the true exeption to this tho. Its fully capable of turning with the 51 and has a speed advantage over it at most alts.
In a K4 vs a P51D 1-on-1 co-E fight the Pony is as good as dead. Any other result means that either the K4 pilot is clueless or the P51D a real god of Aces High. It's not that the 109 can turn with the pony (in fact it can't on the short run), it's the ability to build E that makes the 109 so superior, and the speed that allows it to deny the P-51 the chance to run away it's good just to make that advantage count.
As the fight draws longer ,the bigger the advantage the 109 wins over the Pony. Any maneouver the 51 pulls means burning an energy he can't easily rebuild, while the K4's acceleration can turn a fight's energy tables in a heartbeat. If the fight is fought in the vertical (and any decent K4 would drag such a fight to the vertical plane) the 51 has only one option: dive away hoping for the 109 compressing for enough time for some friends to clear his six...