It should be possible to load the 10,75x68 with 400grs solid to App 2100fps? That equals the .450/400!
Any thoughts regarding barreltwist?

You can load a 400grs , with a lower velocity , never equal the 450/400NE and the risk of poor penetration by shooting big game ! There are load data in the reloading manual of RWS .

The 10,75x68 is marginal for big game hunting , it's a good cartridge for heavy game at short to medium range . The 11,2x72 Schüler is a step up , with a lot of another problems . All this cartridges are compromises .

If you like classic old cartridges it's better to use a 404 Jeffery or a 416 Rigby for a safe big game hunting .