Water cooling will keep the heat at bay but excessive voltage will kill the CPU anyway. Thats why there is a risk in over clocking. Over clocking is really about making budget chips perform over their intended purpose thus the user gets something for nothing. A performance chip on the other hand offers little with the over clock in terms of real world computing but it can certainly open the door to instability and cooked components.
Performance wise my 2500K increases in performance each step up to 4700mhz there's a slight increase to 4800mhz then almost the same performance level at 4900mhz and 5000mhz.
When I say performance wise I'm relating to Benchmarks, Gaming, and Video Converting, your 2600K should performance plateau around 4800mhz, so don't run any higher voltage than you absolutely need to, to get the best of the performance gains you can.
Bragging rights is one thing, but if your performance plateaus out, there's really no reason to shoot for higher clocks, running a clock like that is just shortening the CPUs lifespan for no actual gains.
Even water cooling stay below 1.5v, you probably won't see anything more than slight performance gains past 4800mhz anyway, but don't take my word for it test it yourself.