Would it really be that much of a problem performance-wise? I've never run into an issue with games I've played (though he most demanding ones I play are probably Starfox 1&2) If we're talking a drop from 60 fps on most games down to 30 or so, I think most people would be able to tolerate it, providing that emulation of the new chip could be turned off on games that don't need it. If i remember correctly, picodrive had several options on what chips/hardware to emulate for sound.
I'm assuming the bigger issue is the amount of effort it would take to implement? Of course i'm not a dev by any means so I don't really have any idea.