I do stream pretty often. But you are right, LoL is one of my games. But also new tripleA titles, pretty demanding stuff.
But like i said, i have the i7 5820K @ 4,5Ghz now and i had the 4770K @ 4,3Ghz before that, so i know the load really well. The 4770K is almost the same CPU, just with worse thermal compound inside and slower clock speed, but the difference between them is around 5% when OC`d to similar speeds.
The 4790K is a beast of a CPU, one of the strongest processors that you can actually buy without getting into 1000$+ range. And it can handle 1080p 60FPS @ veryfast without hitting the limit, no matter what game is used to stream.
But dont forget additional load from the browser, teamspeak and other crap in the background - that can add up and actually start hitting your ingame performance.