You know, it's possible the P-51's bigger prop has something to do with it. It has one more blade, each one is bigger, and it has cuffs so there's a lot more inertia involved, as well as possibly more prop wash.
I definitely notice what you're talking about, though. I was bringing the mustang in an hour ago, forgot to dial in rudder trim on final and ended up using 5º or so of right aileron trim plus a little more on and off on the stick. Still, it didn't ruin the landing, it just caught me off guard.
As for trimming and stability in general, I've found the pitch in the P-40 tends to wander up and down a bit whenever there's significant fuel in back. Trimming keeps it more or less centered where I want it but I still have to keep my hand on the stick to keep her steady. Conversely, the P-51 always feels solid in terms of pitch stability, regardless of how full the fuselage tank is.
You must have missed the 100 post, absolutely disastrous thread I started in the P-51 Tech Forum that ended up getting locked over a week prior to your post
In summary, information sought out from 3 separate and reliable sources outside of the forums have convinced me that the effect is quite possibly
a bit overdone in the current P-51 version. It never was a sure thing, or an accusation of incorrectness or anything, just an honest question about an issue which wasn't squaring away with me and was affecting my immersion...but I was apparently unsuccessful in arguing the issue was legit enough to be further looked into, and there was no official commentary from A2A, so I assume any issue with this is dead on arrival. The thread was locked before it got too testy. "It is what it is," as they say and I'm pretty sure it would be considered gauche to discuss it beyond my response to your post
Regarding the pitch stability of the P-51 versus the P-40, I have to be honest, I've never flown the Mustang with the Fuse tank filled to the brim because I've never bothered to cruise long range. According to the manual, when the P-51's fuse tank is topped up it -ought- to suffer from reversability during certain manuevers. Real manual says "with the fuselage tank full, the center of gravity of the airplane moves back so far that it is almost impossible to trim the airplane for hands-off level flight. Also, as soon as you enter a tight turn or attempt a pullout, the stick forces reverse." That leads me to believe it realistically ought to be even worse than the P-40 in longitudinal stability with the Fuse tank filled. If that isn't happenning on the simulated article, well then, don't know what to tell ya...