4-8gb should be good enough for almost everyone. a few of you might need 16gb.
Not at all on ROCK. Not even a little bit. More RAM will just go unused. If ROCK runs out of RAM, Roon will crash. If you are not crashing, no need to add RAM.
Performance wins? No.
Stability wins? Only if you are running out of RAM. With 8GB on ROCK, that’s probably around 300k tracks, or maybe even more.