I agree A 7.5 lb 450/400NE 3" would be a little light for that cartridge, and not what I'd want personally.

Though muzzle energy is part of the equation in being legal for certain game animals. However, it is the combination of muzzle energy and bullet diameter, and Physical make-up of the bullet, along with bullet placement that determines killing power. IMO the killing power of a 450/400NE 3" is far and above the killing power of a 9.3X74R with like type bullets.

That being said, IMO, the 450/400NE 3" in the 9.0-9.5 lbs weight and a 9.3X74R double in the 8.0-8.5 lb weight with a quick detach mounted scope is a fine combination for a two rifle Safari. My combination today is a 9.3X74R double, and a 470NE double, but I'd far rather have a new Heym 88 PH 450/400NE 3" double for the top end of the pair!

Both the 9.3, and the 450/400, in properly weighted rifles, are far easier to shoot well than any of the larger diameter rounds. The 9.3 is not legal for the larger of the big five in all places, but will do the job if the larger rifle is somehow put OTS.

A back-up rifle is not a back-up rifle if it is in the bakki! You carry the larger of the two, and let a tracker carry the lighter one, till needed. Of course, everyone is at liberty to do as it suits them. IMO, there is a lot to reccomend the 450/400NE 3" and the 9.3X74R scoped double as a pair of light, and heavy, unless your main quary is Elephant. In that case then, I have to agree with Will, and would use the 470NE, or my big bore favorite, the 500NE for the top end, but I don't hunt elephant! IMO, most who hunt Africa, will never kill more than one, maybe two ele in a life time, and for this guy the 450/400 is all that is needed on the top end!