After adding to Math::Int128 a new set of operators that use a preallocated argument for output, Math::Int128 becomes faster than Math::GMPz, around 60% faster

That's more like the figures I anticipated (at a guess).
My "I don't see much scope for improvement" comment was in relation to my own enhancements to Math::Int128 which removed the allocating/deallocating you mentioned - and which made the int128 multiplication a little faster than using Math::GMPz's functions (and, of course, significantly faster than using Math::GMPz's overloaded operators).

When I get back to my Vista64 box, I'll grab the latest github version and see for myself ... and, of course, post again with my results for that machine.

Cheers,
Rob