Believe me, I have the greatest respect for those who've met these animals in the wild. I won't for a moment question your judgement on which cartridges are sufficient. I only wish to note that even the experienced have different opinions on the matter. That is the only grounds upon which I haven't taken a specific piece of advice as the final word.
My own background is in the technology that makes these rifles, particularly the semi-autos. The only claim to expertise I have is with regard to ensuring reliability with autoloaders. The most reliable tend to be intermediate in power. But I believe I have struck gold by identifying one that has the dimensional clearances for a large bore round whose strength specs are sufficient for its medium pressures. That's a project I'm currently exploring.
What I'm also aware of is ballistic similarities between certain rounds. For example, the 450 Marlin is equivalent to a modern load of a 45-70. Those who use a heavy 45-70 would not be undergunned for switching to 450 Marlin, though the 45-70 probably has the edge with economy and options. But if dimensional specs and load uniformity are an issue, the 450 Marlin may be the better choice in an autoloader.
If 444 Marlin is insufficient, than it's insufficient. The reason for wanting to use it was that dimensionally it would fit well in an certain AK type action. I would argue that a properly tested and tuned AK action is at least as reliable as a manually operated bolt.
If 450 Marlin is sufficient, which I have a hard time not believing, then the available autoloading option is a customized AR-10. The drawback is that reliability is not equivalent to the AK. For it to be trusted for dangerous game hunting, it would have to be tested in all applicable conditions. A new gas system (Daewoo type perhaps) might serve well.
I would never say in a million years that autoloading or increased capacity can compensate for low skill or insufficient knockdown from each shot. All I'm saying is that reliable autoloaders are not impossible to make, and that's what's my department. Mating reliable designs with sufficiently powerful cartridges is my goal. Have a little faith
In all reality, chances are I'll learn a lot about more about rifle design, build a small fraction of the things I imagine, and if I ever go hunting the most dangerous game, I'd probably use a 458 Win Mag that belongs to an experienced hunter friend who would probably use his 375 H&H; he's a better shot, I'm less recoil sensitive. Alternately I may acquire a 375 H&H myself. Truth is I'm perfectly comfortable with bolt actions and from recreational shooting with milsurps I must say I've developed a pretty good rapid reloading reflex. Developing a big game autoloader would be about making something new and good, not about something I could never live without.
Again thanks everyone for your generous sharing of knowledge. Keep it coming!