John,
Like you have seen the the ballistics of the 500/416 are very simmilar to 416 rigby. In fact they are supposed to be identical, yet with lower chamber pressure making it ideal for double rifles. The only problem with the loading of different bullets is that may dramatically effect the regulation of the barrels. My double rifle expierance is limited to my 9.3x74 over/under so I cannot confirm the effects on a sideXside. Through practice I have been able to consitantly place good shot groups with different wieght bullets by adjusting aiming points. I must agree with you about how beautiful the krieghoff is and from the performance my cz actioned 416 rigby would love a well balanced double with the same punch.
regards, Hal