It depends on the game. Anything using the latest Frostbite engine (which is relatively new but you'll see a ton of in years to come) scales well with the i7's extra threads. You see the old i7 2600K matching or beating an i5 6600 or even 7600 in titles like Battlefield, Mirror's Edge, Need for Speed, Mass Effect, etc., and within the same generation the i7's are holding a ~25% performance advantage over an i5 at the same clocks, which is a lot more than you're going to get from any OC. Couple that with the fact that a stock i7 will draw half the power of a heavily OC'd i5 and can be used with a cheaper motherboard and it's pretty damning for the 7600K.
I won't drop a ton of benches here but you're welcome to look them up. Here's just one:
Truly multithreaded gaming is only just arriving, but it's here, and you're going to see chips with more than 4 threads having a reason to exist in a gaming PC from here on out.