Having fired exactly TWO follow-up shots on someone else's elephant being my total experience with shooting elephant, you can take what I say any way you want.

I do have some experience with the 450400NE, and with that in mind combined with what I know about what is needed to stop a full charge, I will say the 450/400NE with a good solid bullet, has as good chance as anything larger. Once a charge starts, in ernest, nothing short of a CNS hit will stop that charge for good. Any cartridge with enough pennetration to reach the brain will stop itThis is assuming the bullets hits the brain.
I would say the likelyhood of a bullet of less than a 577NE stopping it with a knock-out with a close shot near the brain to be doubtful. .
This is not to say the ele might not be turned, because I find just about any real solid slap the the ele's face will turn him. This gives the double man a chance at a side brain, or heart/lung shot as he turns.

This said, I don't think any chambering can be a guarintee in it'sself. I personally would not hesitate to hunt jumbo with a 450/400NE double. Now if you ask me what I would prefere if Elephant hunting is all I would do with a rifle, I think the answer would be a double rifle chambered for a 500NE!

All only opinion!