Are the DLLs produced by compiling XS code with MinGW compatible with a Perl built with MS VC++?

Yes they are - and vice versa, too. I've often mixed things in this way (in both directions). Exceptions may exist - such as Inline::C and Inline::CPP, where you've built an extension that expects you to be using gcc (or cl) and run it with a perl that expects you to be using cl (or gcc). If the VC++ compiler in question is other than version 6.0 then you might strike an additional (very rare) problem because of different runtimes being used.

(I also wondered what Strawberry will achieve. Having read this thread, I can't see that it has anything to offer me ... but that's about all I've been able to deduce ...)

Cheers,
Rob