Hello. I would like to guys if I will get any improvements moving from 4c/4t to 6c/6t. I3 8100 to 9400f. I have RX 580
Certainly. 2 more cores, higher clocks, more cache, faster memory support = significant improvement across the board.
It is always best to provide what games you play or want to in the future. The Polaris will be the bottleneck the vast majority of the time. If you are a heavy Battlefield 5 player or another title that can hammer the CPU threads, then the i5 will struggle bus it at times too.
I say go for it if the i5 is a good price, particularly if recouping part of the cost by selling the i3. Otherwise, yeah, time to upgrade the platform.