You could think about the .400 Tembo - it is a slightly shortened .404 case firing .40 calibre bullets (not .423 like the Jefferey or .416). It fires a 350grain Barnes X at 2300fps (SD .312) and 400grain Hawk or Rhino at 2200(SD .356). Designed by Kevin Robertson (writer of Africa's Most Dangerous) and Ed Woslum (Evolution USA - www.evo-rifles.com).

Probably too new for factory rounds as yet, but no doubt Superior would make it. It would be a good choice if you didn't want a belted round like the Taylor (although personally I don't see anything wrong with belts on the right cartridge). It would be an easier fit than a .404 on a standard Winchester M70 action because it is slightly shorter, but that isn't a factor for the standard Mauser which can take a .404 Jefferey. Krieger makes .40 barrels too.

Another option could be the .425 Express, a .300 Win Mag necked up to take .423 bullets (same as .404 Jeffery). It is reported to do 2300fps with 400grain bullets. I imagine that the recoil would be pretty stiff though and I'be no idea if factory rounds are available (Superior again?).

Of course if you are going the "whole hog" and having the rifle built, there is always the .400 Holland & Holland, but I think that is longer than the .404 Jefferey and I doubt if it will fit a standard Mauser action (could be wrong though - never seen one!)