Well what game, what do you use to stream, what quality in game, what quality streaming
There are so many factors that we cannot tell you the fps you will get.
For me in CSGO on Medium setting in 1080p(to reduce input lag since I play competitively) I only lose like 5fps while streaming at 720P 60fps. Which isnt much saying that im getting close to 300fps.
I Use x264 as my encoder in OBS. I used to use NVENC but x264 has better stream quality.
My cpu is a 4790K at 5ghz and a 980ti with factory overclock.
CPU usage while streaming and gaming is about 85% and GPU usage doesnt matter since im using my cpu for encoding the video