D8NzZ

Only upgrade if performance does not meet your standards. 
, Do you mean 2560x1440? For 1440p 144Hz, the best GPU you can possibly get is what you should be after. At max, it's more demanding than even 4k. 

A 1070ti is still a very good GPU today. You should upgrade the GPU if it doesn't perform as well in the games you're playing rather than upgrading it because of a bottleneck. If your games perform as well as you want them to perform then there's no point to upgrading the system, however if the performance is too low, or the game lags and the issue is the GPU then sure, upgrade.

What type of encoding do you utilise when streaming? What bitrate are you attempting?
 
If you're using x264 try playing siege in 1440p /higher graphic settings as this will offload the gameplay to the gpu