XFR to me is a gimmick, except for mobile users, and maybe, just maybe, standard Joe Bloggs.
Clocks are limited as it is. Keeping constant FO4 is fine but the problem with a 14nm high clock design is the tiny wires, RC not scaling, major EM issues, DP complexity, huge fin variations affecting current, end to Dennard scaling meaning high current densities, so high localized heat causing with ~30C deltas, and Vt variability being a significant showstopper. Plenty of research papers have discussed these at length.
IBMs J Warnock had a research paper discussing this based on their own 4-5.7GHz chips. And these are watercooled/chilled watercooled chips still facing these issues.
If XFR is dependent on average chip temps, that might work well, but if a single hotspot kills XFR, then it wouldn't be much use except a benchmark winner.
Voltages for Turbo and XFR clocks are also key here.
Sent from HTC 10
(Opinions are own)