The 460 derives directly from the SuperMag concept. Same length case. S&W went to Dan Wesson in developing it. The SuperMag concept was to take the existing cartridges and lengthen their case to 1.610 inch. The 460 is a 45 Colt case lengthened 1.610 inch. Dan Wesson called it the 460 SuperMag. S&W called it the 460 S&W for obvious marketing reasons. So yo use I’m not comparing the 460 S&W to a 45 SuperMag, it IS a 45 SuperMag from the get go.
As for the 445 SuperMag compared to the 454 Casull, it gets a bit more complicated. To start with, the 445 was intended for a very specific purpose, silhouette shooting. This is rather specialized niche and the 455 will produce a flatter trajectory with less recoil than the 454 casull for the same delivered momentum at 200 meters. In the terminal ballistics for this game, momentum counts, energy or caliber doesn’t.
That said, the 445 SuperMag is obviously a good hunting round. It certainly outperforms the 44 Mag. It’s difficult to compare to the 454 because you also have to drag in questions about what sort of firearm you are putting it in, what you are hunting in what kind of terrain, etc. Not that there is a great difference between them, but sorting out what difference there is drags in such questions. For starters, one might compare them in a single shot pistol since then it would be possible to compare them in otherwise identical arms. If I compare the muzzle ballistics from a single shot the 445 SuperMag and the 454 Casull are a pretty close match. It’s also interesting to note that this comparison reverses the old 44 Mag vs 45 Colt comparision. It’s always been pointed out that the 45 colt will do what the 44 Mag will do with less pressure. In the present case, the 445 SuperMag runs less pressure than the 454 Casull.
BTW: I have no particular bias here since I have both dogs in the fight. I'm not planning to get rid of either.
On the other hand, you made the remark about why not just make the 454 with interchanagble barrels. The Dan Wesson product line is somewhat limited at the moment but if they put out a 454 I would serioulsy consider buting one. No body else has seen fit to play with the interchangable barrel concept. (The S&W X frame barrel can be swapped out relatively easily if you have the special tool, but it does not allow for an adjustable cylinder gap like the DW. And lately S&W has not been getting the highest marks for getting cylinder gaps just right.)