There is absolutely nothing wrong with the 458WM. With current powders it easily reaches the velocities of the old Kynoch loads and in many cases beats them. I have chronographed many original Kynoch loads and most actualy show 150-200 fps lower velocities than thos published. The current 458 loads do exactly what Jack Lott originally intended his version to do. Personally I have never seen where another 100-150fps made any difference but if that is what you want then the Lott is a fine round. But if you really believe extra velocity is advantageous then why not just buy a 460 WBY and be done with it.
In the past fifty years thousands of African hunters and PH's killed tens of thousands of head of big game with it and the vast majority will tell you it worked great then and is even better today.
I'll bet there are currently as many detractors of the 375H&H now working in Africa as there are detractors of the 458.