Ive had mine at 3.6gig but with this warm weather it gets abit toasty, so it back down to 3gig with 45 degrease on load with a asus silent square pro at 3.6 it was hitting 60 degrease core temp,1.4 volts.
its been running at 3.8gig 1.65v on water for ages (almost 2 years) and no ill effects so far... not reccomended for those who can't afford to replace the CPU tho.