Remember the 9.3 was designed from the start for big African game and is legal in all but two countries for the big five. The 9.3 was Germany answer to the 376 H&H when loaded to it’s full 63000 psi potential it equals the 375 in comparable bullet weights without the belt nor magnum length action. The real advantage of the 375 is that long case and much lower pressure which matters in Africans climate at 30+C and near sea level you want all that case space and slowpowder burn to keep peak pressure down.that’s a why when I venture to where it’s not and sticky the 375 goes with me. The 375 with 300gr vs the 9.3 wt 286 gr can be loaded to nearly identical velocities and joules of energy. The key difference is peak pressure the 9.3 and it’s 30/06 based case can take 60000+ psi the HH not as much but much larger case for more slower burning powders. I use RL17 in the 9.3 and RL22 in the big Brit.
True but the 9.3 was developed in the early 1900s. The .358 NM was developed in the mid-1900s. It is not surprising that it might be a more capable bullet. Just like it is not surprising that a .338 Lapua developed in the late 1900s (or the relatively new .338 NM) can outperformed the .358 NM. Bullet development has improved with time.
The fact that the 9.3 is still legal for the big five is a grandfathering provision more than a measure of its capabilities. Correct me if I'm wrong but new calibers less than .40 cal are being not approved for the big five despite their capabilities which may well be in excess of the 9.3.