I think Windows Defender is a fantastic line of defence and it's definitely better than installing garbage from Trend, McAfee etc. That said, Lenovo, HP, MSI, Dell etc still preinstall crapware on their new machines from Norton or their ilk threatening that my machine is "at risk" if I don't pay them money.

I wouldn't turn it off unless I knew that I was only installing games from Steam. But if I did I think performance would improve. A game from Steam could still contain malware so you have to exercise some common sense. Even on SteamOS a game could be malicious but since its containerized the scope for damage is limited but not necessarily impossible to break out.