1)If you can, that is. 120GBs is pretty tight for an OS only drive.
2)It doesn't have to be the same company/sticks. What matters is that they are packaged together; all the compatibility testing was done already.
I would suggest bumping it up to a 2x 8GB kit though. Some games are pushing the boundaries of what 8GBs can handle - some have already exceeded that.
Memory behaves in a similar manner to storage; it too, slows down past a certain point - say, 75% or so. So even though you 'have enough', you never really want to use it all.
3)As long as your RX 580 isn't thermal throttling, then it should be fine. It has a temp limit of 90C, after which failure to control will make the gpu, and PC, shut off on you.
I would strongly suggest setting a custom fan curve if you haven't already - the default ones tend to suck most of the time.
Do not pay any mind to those 'performance' statements. Userbenchmark is worthless for measuring a PC's overall performance. Your RX 580 is being compared to people who overclocked theirs 'to the moon', instead of a stock gpu.
It's still within 10% of those heavily OC'd ones, so it's performing fine.
It does have its uses, though, such as troubleshooting.