The primary mode of 'yaw hunting' was in a near terminal dive when a lot was going on aerodynamically. The ventral fin was thought to be the solution - and remember that the kits and the production D-10 with 'fin' installed were arriving at about the same time 100/150 fuel was at the depots in England - with a corresponding boost on Hp and torque.
Rudder boost was initially tried on the late model B, and when a few more tails continued to be snatched off in high speed rolls, a reverse rudder boost tab was installed to make it harder to push those rudder pedals.
Short answer - anecdotal - I never had any time in the B and my D time was limited to medium speed aerobatics so I can't compare personally.
I do know that all B/C's in the 8th AF inventory received a field mod ventral kit installed at the Base Service operation - didn't require Depot level technicians. I have talked with pilots old and yound who flew both, including my father and his sense was a.) the 51D had slightly more yaw wandering than the B... but saying this he pointed out that the 1650-7 in the P-51B-10 that he first flew in combat ops was restricted to 67" Hg versus the P-51D-10 that he got two months later - at 72-75" boost.
I did get a ride in a P-51H with dual controls and my sense is that the 51H was definitely more stable in yaw and had no dutch roll tendencies.
