Velocity of the 10.75x68 seems to be approx 2200 to 2250 fps with a 347 gr bullet.

Velocity of the .450/400s with a 400 gr bullet is approx 2100 to 2150 fps. Calibre .407 to .411.

The old .404 Jeffery. .423, reported a 2170 fps velocity with a 400 gr projectile. Nowadays msybe 2200 to 2350 fps. The old and new .404 has a good reputation.

The 10.75x68, .423 calibre was reported as a failure on DG. Eg Harry Manners, Kambaku. Who switched to a .375. The principal claim to the failures was bullet construction.

Is the modern 350/347 gr Woodleigh Weldcores and FMJ better constructed? Almost certainly. Is 2200 to 2250 fps adequate? I would think so, but marginal decreases also exist.

Can a 400 gr .423 bullet be loaded in a 10.75x63 or x68? At what velocity? If 2100 fps or even 2150 fps? Zero practical difference to the old .404 Jeffery.

I will edit this post to discuss equivalent sectional densities.

Is the barrel twist rate the same?

All armchair stuff, given the lack of field users.

But I like it's lack of reputation ie the x68. Maybe get one nice M98 from Germany one day cheaper.