THis suprises me that some are seeing hard cast bullets with gas checks and getting lead fouling problems. I have for years used "QUALITY" hard case bullets from Cast Performance with gas checks in big bore rifles at velocities up to 2100 fps but usually around 1900 to 2000 fps. In fact my "North American" 458 Lott load uses a CP 440 gr loaded to 2000 fps and I have never had a problem.
I have seen some cast bullets that were to soft and with poorly fitted gas checks cause some problems.
I believe the question is asking about lead tips on jacketed bullets and what happens to these bullets in flight, not in the bore.
Most of us have seen high speed video of bullets in flight going through something like an apple or something like that. One I remember was a soft point out of a 223 Rem. I believe it was a Hornady 55 gr SP loaded to 3200 fps and just before it impacted the apple, it was clear that there was still a perfectly shaped lead tip on the bullet.
All lead bullets should not be used at 3000 fps for one simple reason, the metal used in these bullets does not have the integrity to hold the lands of the rifling without shearing off. That is why rifles designed for lead bullets will generally have much deeper grooves in the rifling so the bullets will hold the lands better. Modern high performance rifle bores have relatively shallow grooves as the copper alloy jackets will hold the lands much better and do not need the deeper rifling.