I asked the same question last year. I had a successful African plains cull hunt using the .270 Win. with Nosler 150 PTs to take ten animals, and the .338WM with 225s to take one. These calibers were very effective, but after thinking over the various shot presentations and results, I decided a commercially available cartridge which was flatter shooting than the .338, harder hitting than the the .270, and effective out to 800 yards would be even better.
After discussing this with Kirby Allen, we decided the 7 STW would be an excellent choice for the purpose. Of course, for handloaders one of his magnums would be even better.
BTW GG, my hat's off to you (and/or your client). A 756 yrd. shot like that with the .270 is very, very difficult.