Well I was very confused to see, that the thread got 3 pages now :o . From my experiences with my old system (6 Core X5650) I can tell you, that I've got a FPS drop from ~10-15 FPS per encoding job (I've used NVENC 25MBit/6Mbit for recording/streaming), so almost ~20-30 overall. Streaming with x264 was no option, because my old CPU was at a to high load, especially with games that are using 6+ cores (Watch_Dogs 2, Battlefield 1).
With my new setup (i9-7900X) I made some streams with recording at the same time, I hope the logs will help you, maybe you find some mistakes too? So, actualy I've made no gameplays with games which use all CPU-cores, because Battlefield 1 won't start anymore since I've reinstalled windows (1-2 seconds of loading in windowed mode, then it closed without any errors/messages and I'm back in origin ... biggest fail of EA ever) and for Watch_Dogs 2 I've got no time until now. I hope to find a little bit of time next week, atm there are to much betas which I want to play D: .