Mine's running like peaches & cream at 4.4GHz (all 4 cores synced).
Temps will go just above 70C when running the Intel Extreme Tuning Utility benchmark, hitting 1014 marks on XTU, yet still stable as a rock, on the ASRock Z97 Extreme6 MB. Don't know why the UEFI has a warning about pushing further, as I do sometimes with XTU & all is fine.
Maybe since the MB was from 2014 (although still one of the best Z97 MB's & the 1st with Ultra M.2), it doesn't have the capability to know how far the CPU can be safely pushed. I do know that it doesn't stay in tune with with the graphics, the OC settings are lower than mine out of the box.