Spend an hour talking to junior dev about not nesting multiple #ID selectors in sass. He is still convinced it's better for 'rendering performance' and refuses to change it. Should I ask him to add 200 elements with the same style on one page to illustrate the problem?

Of course! Then tell him that if he's in a scrum team and don't listen to his seniors... well, he'll learn to listen in the next team 😧

Yes.

It's first time I lead junior, don't wannt to mess up, will try to give him real live example so he sees why it's not a good idea

How much SASS is there..? You're unlikely to notice any real world performance difference, especially when everything is compiled and minified... The number one rule I follow for any project is optimise when the project is finished, early optimisation leads to unreadable code.