Ballistics expert Keith Luckhurst ran some trajectory tests comparing a .404 Jeffery loaded with 400-grain bullets at 2280 fps, a .458 Winchester Magnum loaded with 500-grain bullets at 2090 fps, and a .375 H&H leaded with 300-grain bullets at 2550 fps, all sighted in at 100 meters. According to Luckhurst, “At 250 meters the .375 Magnum has dropped 11 inches, the .404 has dropped 13 inches and the .458 has dropped 18 inches. But at 150 meters there is a spread of only one inch between these calibers, and at 200 meters it is four inches. Most gunwriters would describe the .375 Magnum as flat shooting and the descriptions of the .458 tend to include words like ‘rainbow trajectory.’ In reality, the point of aim for any of the rifles is virtually the same out to 150 meters.” Luckhurst concludes that the .404 Jeffery, with better penetration and less recoil than the .458, a trajectory almost as flat as the .375 H&H, and overall performance similar or equal to the .416 Rigby, is a particularly well-balanced rifle for the largest and most dangerous game.
I would stay with the proven load of the 400gn bullet at 2300fps with 84gn H4350