For the transmission of videos, how much better is i7-8700 vs i7-6700?

You mentioned two things here, rendering and streaming.

For the representation, the update of the CPU could be a great factor for improvement, but it depends on the neck of the bottle from now on. If your IO is the bottleneck, updating the CPU will not help and vice versa.

For the transmission, the more RAM is better, since more files will be stored in RAM and less IO will have to occur for each file transmitted.

Are you experiencing any specific problem that is causing you to update or is everything working smoothly with the current configuration you have? Or are you starting?