Got off work early today and spent the afternoon reading the Speer loading manual. The .458 Lott beats all the cartridges discussed so far in the 300 - 350 grain velocity range, and also the 500 grainers. It's up there with the 470N.E. in velocity with 500 grain bullets - just a bit thinner!
I'm guessing recoil is an issue? The CZ 550 African is a bit less than $1000 in this caliber. A bit bulky, but a good deal as far as Safari rifles go.