I had a custom loop back in the day - that I really miss - but that's besides the point. My i5-2500K hit 5.2 stable and I never really pushed it higher because I was unsure of how hard to push it. I did boot stable at 5.3Ghz. Overclocking is going to greatly depend on a number of factors like the efficiency of the loop, how stable the PSU is, motherboard quality, etc.
But usually the 2500K can hit 5Ghz on water. It was frequently reported to hit that on air.