The SYNC'ed SE is for me a worthy product because you don't get only the MIR reverb but also loudness matched samples. You don't have to fuss around any more to get a reasonable realistic sound. IMHO you get the room sound, balanced samples and a more convenient UI.
The new plucked instruments the issue of sound balancing is less important. If you don't care for the UI you get the room sound for 20,- (if you already own the lib). Here I agree, paying for a MIRx extensions makes more sense than buying a SYNC'ed version of everything.
That said I like the Synchron Player. So now that I have SYNC'ed version I want to use MIRx extensions for them. After all they were recorded dry anyway, Yes, I could use MIR PRO. But that's way to expensive for private use.
So, yes for a MIRx Synchron Stage extension and yes to bring MIRx to the Synchron Player.
PS: Anyway, I don't beleive you see a MIRx Synchron Stage unless all old libs have been released SYNC'ed.