just fwiw, this was discussed a bit in the shoutbox
however, from a MAME point of view it's still a bit 'backwards' and relying far too heavily on system specific stuff in a CPU core, rather than improving MAME / the CPU core as a whole
doing it properly will have a much more significant performance impact, so don't worry too much about that
since it will likely require a z80 rewrite, and the ability to slow down the CPU on the fly etc. a proper solution is likely some way off, so personally I'd quite like to see something like this as a temporary measure, but IMHO to stand any chance of being accepted you'll probably need to make some C++ derived 'specz80' CPU type with and use virtual functions + overrides to keep all the speccy specific code as far out of the actual Z80 core as possible.
that said, there's a fair chance the devs just aren't going to accept this kind of solution at all, that would not be my say.