I agree - the 'sabot' has limits, but of course, the limits are closely tied with the material being used. The harder the sabot material is, the thicker it can be, I'm sure.

In the .458 scenario, they were making 4,000fps with 220gr. RN bullets. The whole idea (I assume) was to break 4,000fps with a .458 WinMag. using a reasonably heavy bullet. I doubt that bullet held together for very long, but hard to say. Barnes TSX are available in 250gr., but I suspect the friction would be too high, to get much over 3,000fps with them.

A still like shooting bore size, or almost bore size in a shotgun, rifled or smooth.

I freely admit the sabots might have some military applications, for sure - a 12 or 10 bore platform makes for a LOT of diversity in projectiles.