IMO the IIIx is a big step-up over the IIx, primarily because of the ribbon tweeter, less so the extra bass. But, I have never heard the 2.7's.
I have done all sorts of mods over the years but in all honesty nobody could tell, just no sonic difference. There were measurable differences with some mods, but by and large nada. I also found replacing the coils (inductors) with heavier units offered the closest thing to an audible benefit short of active bi-amping (bypassing the crossovers).
$850, I saw them for ~$300 a while back, gee maybe I should consider selling mine... <pause> Nope, the 3.7's are pushing $6k, I'll keep mine a while!
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley