I think you are confusing the P47 with the 51. From what I remember, pilots said that the 47's stick was hard as if stuck in cement and some P-51 pilots said that they were slapped by the stick in high speed dives.
Both conditions are true. Before locking up the stick would flail about violently. The P-51 was a death-trap in an unrestricted dive compared to both the 47 and 38 and should never, ever exceed maximum dive speed.
The following was written by P-47 pilot Charles D. Mohrle:
Upon entering a vertical dive in a P-47, rapid acceleration was inevitable and, at high altitude it was exaggerated. At about 550 MPH indicated, buffeting became evident and it rapidly became severe. The control stick jerked violently in all directions and it was necessary to hold on firmly, with both hands, to manage it. Quite suddenly, the buffeting stopped and the stick became rigid - as though set in concrete. At that point, you jammed both feet hard against the rudder pedals and pulled back on the stick with all your strength - still with both hands. And you stayed that way, simply waiting for the airplane to respond.
In the vicinity of 15,000 feet, depending on atmospheric pressure, the nose of the airplane began to slowly creep toward the horizon. The rate of movement increased as altitude decreased. Level flight could be restored by 8,500 feet if you were willing to absorb the G-force punishment. In my last two dives, I pulled out gradually attaining level flight a bit below 4,000 feet with the airspeed needle still on the stop.
I'm not an engineer so cannot speak with technical competence, but I know of only one other propeller-driven airplane capable of such unrestrained dives - the P-38. And the Lightning had a tendency to "tuck under" early in the dive which made for a greater loss of altitude during the pull-out. It too, experienced buffeting.
I had a good friend who commanded a squadron in the South Pacific. They were equipped with Thunderbolts for most of his tenure. With the arrival of B-29s, their group was assigned escort duty and furnished with P-51s because the longer range of that fighter allowed them to stay with the big bombers all the way to Japan. They lost four pilots before learning that the Mustang couldn't be dived with the same impunity as the Thunderbolt. In each case, the left wing folded up and over the canopy, making escape impossible.
