I think some perspective is in order here. Check out kweassa's post on turn rates and you'll see the P51-D and B feature in the bottom 1/4 or so of the list. The P51 can out turn a number of things, but it never was, and never will be a stall-fighter. If you want to prevail in angles fighting try a spit mk8 or 16 or a zeke or N1k.
If you still think the P51 is a mongrel, then (and im quite serious when i suggest this, as its exactly what im doing this tour) fly a P47-D or N for a tour, then your (and my) beloved pony will feel like it handles on rails.
Also try the P51-B or the P51-D with only 4 50 cals, and only take 50% fuel and the drop-tanks. Drop ya tanks when ya need to get jiggy and fly it to its strengths and its a real monster.
When you say it doesnt turn like it used to, then nor does any plane in the plane set. AH flight model has been through numerous changes over the years, some small, some not so small. But all the changes will always apply across all the planes in the planeset at any given time, so any planes relative performance to others wont really change. Further i dont think HTCs latest change has altered its characteristics much at all.