Improving gaming performance while streaming

Currently on a R9 Fury X and a 5820k with 16gb of ram on windows 8.1 ltsb.

I am streaming to YouTube at 1080p60 on the amd encoder. I play most games at 1080p with a 144hz monitor

I would like to know if there is any performance gains in game if I were to buy a used 970 or 1050 and had obs use that GPU to encode the stream.

Alternatively, should I upgrade from my R9 Fury X to Vega (when the 7nm refresh drops) or a used 1080 (if Vega becomes another disappointment)?