Hey,
Intel can and does disable overclocking just using Windows alone. The microcode update is pushed via Microsoft Updates so it's pretty hard to stop. Even if you do not update the BIOS, and set an overclock in the BIOS, when you go into Windows 10 you may discover it simply didn't work.
Get the i5-6600K, as well as a suitable MOTHERBOARD and reasonable quality POWER SUPPLY.
As for GAMING, the i5-6600K can make a noticeable difference, however it will depend on:
a) GPU, and
b) GAME, and
c) SETTINGS chosen (resolution etc)
Performance difference?
i5-6500-> Turbo value drops to about 3.4GHz under gaming load
i5-6600K-> Can probably hit about 4.4GHz (depends on silicon lottery, motherboard, CPU cooler and power supply)
44/34 = 29.4%
The performance in games as said will vary, but 30% is about the best-case. You may see over 20% in some titles, but most of them will be minor.
I would personally spend a bit extra. When I got the i7-3770K I thought the extra $100 to get HT would prove wasted money. I do not regret it now. It helps saving video editing time, and with DX12 coming and games slowly getting more demanding it is extending the life of my PC.
Other:
It's not just the average. Minimizing the LOW FPS drops improves gaming. It may only add a couple of FPS but it will help.