Good point that we sometimes fail to analyze things, instead going on emotional reaction or vague rumors.
So... with that in mind, I'd like to contribute a few observations:
First, I don't think that the .458/round nose vs. .45-70 cast is necessarily an invalid comparison. After all, that's how these two rounds came into being! I'm sure that many if not most .458's are set up to feed round nose bullets, as that's what's available, and .45-70's generally set up to feed flat-nosed cast (or jacketed), again because that's what's available and that's what's correct for the guns.
There are a few flat-nosed solids for the .458 but not many, I have seen pictures of Trophy Bonded and I believe that they had a nose flat. Now that would make an interesting comparison, the .458 with a flat-nosed solid, let's see how far that would penetrate and would it be a significantly greater wound channel than the round-nosed bullet at the same velocity?
Or, let's crank up the hard cast .45-70 bullet in the .458 and see what happens. Point two... quite often a little less velocity makes a bullet penetrate further. Garrett's 500+ gr. bullets are moving well below the speed that it would take to make them expand, so they keep their profile and don't lose any energy due to deformation.
Crank up the hard cast to over 2,000fps and I'd bet anything that it will penetrate less, due to some nose expansion.
More evidence that 'less may be more' - note the amazing penetration of the ~350gr. .45 Colt bullet at 'only' 1400 fps. I'm quite pleased by that as it is similar to the bullet that I shoot in my revolvers, although not nearly that fast.
So... that would lead to the conclusion that if you download a .458 to .45-70 velocities with a hard cast bullet, could it be MORE effective (as far as penetration) than the factory .458 round nose solid? Seems a little hard to believe but it is really not a matter of one cartridge being superior to the other, more that they each have some different characteristics due to the types of bullets that each commonly shoots. But as each cartridge commonly has certain ammunition available, it is still a fair comparison to pit one against the other with the available loads, in their respective guns.
Well.... that ought to stir things up. Oh and one note on the extractor test: I believe that the Mauser extractor will work better in the gun than out of it. While holding a round in the chamber, the Mauser extractor should be supported by the barrel and receiver wall, preventing it from pulling away from the rim. The Remington extractor is of course supported by the bolt face whether it is in the gun or not, so it should perform the same either way.
By the way I'm not taking sides in the extractor debate, I believe that both are perfectly good designs, and under normal circumstances, the first thing that should fail is the case rim, not the extractor. The mauser design would be quite a lot easier to service in the field, however.