I have a Krieghoff Teck O/U double rifle on order in this caliber.

According to DEVA (the German Research Organization for Hunting and Sporting Ammunition) the 8X75RS compares with the 8X68S as follows:

8X75RS @ 3880 Bar pressure: 220 grain Hornady @ 2591 fps. Water capacity 81.76 grains/5.30 grams*

8X68S @ 4400 Bar pressure: 220 grain Hornady @ 2804 fps. Water capacity 86.54 grains/5.60 grams*

In other words, you would sacrifice a little over 200 fps. for a rimmed cartridge suitable for a single shot or double rifle, with more than commensurate reduced pressure. A 12% reduction in pressure would result in a 7.5% reduction in velocity.

I would consider either load a good competitor for the .300 H&H, and a better choice than the .300 H&H as far as bullet selection is concerned, and the 8X75RS as a cartridge much more suitable for a double rifle.

This, despite the fact that I have killed over 90 head of African game with the .300 H&H.

The 8X75RS brass can be formed from 9.3X74R cases.*

* The Handloader's Manual of Cartridge Conversions