Otto:
If your ex-16-bore Sauer will handle the full nitro load at 17 tons, there's a good chance that the Sterlingworth will handle a 'nitro-for-black' load at only 9 tons. Having said that, I wouldn't be comfortable without a third bite of some kind, and the Greener cross-bolt is unarguably the best.

Sportingrifle:
You will note that Ellis-Brown will not build double rifles on any American-made shotgun actions. There is probably a good reason for this!

Its just a thought, but I suspect that German shotguns in general might well have finer firing-pins more suited to the rifle conversion. In my experience, many American shotties, most Belgian clunkers, and pretty-well all hammer-guns have oversized firing-pin holes which would definitely need bushing to be safe.

You may have already seen this (everyone else has!) but here's a web-yarn about a .577 light nitro built on a shotgun action, for your interest: Click Here. I find myself carrying that rifle afield more and more these days: Hunting Yarn, and another. Hope you guys have as much fun with yours as I have with mine!