I tried it in Fallout 4 - no real difference. Also in Battlefield 4 MP, introduced big FPS spikes. Now I was streaming in BF4, so I turned stream off, but even in the Test Range and not streaming, I got some lag spikes.
I think Game Mode will be terrible for streamers anyway. Taking resources away from OBS or whichever program you are using for streaming. Any game that uses 2 processes could also suffer I think. E.g. Skyrim with ENB, if resources/CPU time being taken away from the ENB process.