I know people called "Louis" (silent S) and also "Lewis" (pronounced S).

Over yonder, both pronounce the S. The best example would be the capitol St. Louis. Still pronounced with the S. Though when talking about historical figures, then yeah I'd pronounce King Louis of France where the S is silent.

Maybe it's just me, but I'd always thought the rule where you drop the S here--instead of Joss's, you'd do Joss'--isn't related to pronunciation. It will still be said aloud 'Jossis', right? But that it reads as less ungainly without the extra S clogging up the works.