Between the two calibres which you have mentioned , l would personally opt for the 9.3 millimeter mauser in your case , because as you say , you cannot handle the 400 grain bullets in your .416 Remington magnum.
Speaking from personal experience , many of my continental clients used to use a 9.3 millimeter mauser bolt operation rifle to secure Gaurs . A good 286 grain soft nose bullet fired from the side , which pierced both the lungs of a Gaur and opened up nicely inside them , would guarantee a dead Gaur. A beast hit in such a manner would go eighty to ninety yards , blowing blood from it's nose and mouth , before dropping dead .
Now , a 9.3 millimeter mauser cartridge was rather a little bit on the light side to stop a charging Gaur above 1800 pounds . However , you are a client and you do not need to worry about stopping charges . Instead , you should focus on placing your first bullet in a vital region . If you can do that with a 9.3 millimeter mauser , then l can guarantee a successfully secured buffalo .
I think that you will be fine with a 9.3 millimeter mauser . It is better to shoot properly with a comparitively smaller bore , than to use a larger bore which you have trouble using comfortably .