I was reading today about the upcoming Kaby Lake CPUs and learnt something about the K series CPUs in both Skylake and Kaby Lake series.
Not only are these unlocked for overclocking, but they also have a much higher TDP.
That means more power and more heat, but it also means they will perform better without throttling under load.
The 6600K passmark score is around 11% higher than the 6500, so it could be worth it depending on the price.