To answer thoroughly I'd have to re-read and test more than I find myself inclined, so all I can give you is some hand-waving. You wrote:

Since Inline::C and XS are essentially one and the same thing ...

Careful there. They differ quite a bit in some formalities, especially in parameter passing. That's what we're dealing with here.

So ... I guess one remaining question is "how come I have to call both dXSARGS and XSRETURN (whereas Robert Hyde's GMP code didn't have to do that) ?"

As far as I remember you normally call neither dXSARGS nor XSRETURN very often yourself. That's true for Inline as well as for bare XS. RETVAL is only for a single scalar return value.

Speaking of which, why are you doing this the hard way? You have to return exactly one scalar (the modified object), so why not declare the method that way and leave it to Inline?

Anno