No idea what they can run at safely, but even at 4GHz mine weren't much cooler than at 4.2GHz. When I did my 20 pass Linx run at 4.5GHz, I sat a 92mm fan over the sinks and they were hot but I could leave my finger on them.

Pain threshold in humans is ~43 - 45*C so burning to the touch is really not that hot in electronic component terms. 60*C is like instant pain.