Daryl,
The .458 bullets are good on heavy game as shown by the ele taken by my 1886 45-90 with 450 grain bullets at 2150 fps.
However, the .400 bullets have also taken a lot of heavy game in Africa. Have you read my M 1895 Winchester .405 story about taking a cape buffalo with 400 grain .405 hand loads?

Also, this double shoots 400 grainers very accurately - one MOA, and when the weather moderates a bit, I will get back to tuning to regulation grouping of the 400 grain Hornady and Woodleigh bullets.
I will be taking the double on a spring bear hunt in New Brunswick in May this year and will use 300 grain bullets. If all goes well, I will post a story and pix.