Did you ever shoot any game with the 405speer at 2100? Excluding dangerous game, I bet the exploding 405 would inflict massive damage and clean kills on Elk, Deer, etc.
No, I never shot game with the Speer 405gr @ 2100fps, just some things. Massive wounds, yes, I would expect that. Clean kills? Perhaps not so much.
Hit in just the right spot, probably. His somewhere else, where the bullet would need some penetration while holding together,? perhaps not.
I know of a case where a deer was hit by the Speer 405gr @1800fps. The bullet worked, and failed.
Deer was hit in the spine, and dropped, DRT. However, only the back half of the jacket was in the deer. The core had gone on, never recovered. The game was cleanly killed, but the total core separation is considered a failure, and had the bullet hit somewhere else, its tough to say if the performance would have been adequate. Up the speed to 2100fps and acceptable results become even more unlikely.
Every jacketed expanding bullet operates within a range of velocity, and the upper and lower limits are defined by its design and construction. Drive a bullet several hundred feet per second faster than what it is built for, and you no longer have a
controlled expansion bullet.
Depending on details and especially the velocity, you could have a violently expanding bullet that opens up under light resistance, OR you could have a hyper violently expanding bullet that literally "explodes" when it hits ANY resistance. This kind of performance may turn a heart/lungs into soup with a broadside shot, but can also blow a plate sized chunk of meat from the shoulder and go no deeper.
Drive expanding bullets too fast and their performance gets...erratic. Sometimes this will still work in your favor, Sometimes, it won't.