This morning

Yes, we can expect more extremists to run under RCV. But also more centrists (e.g., Bloomberg). Since the centrists are closer to the median voter, they will defeat the extremists.

The evidence I have seen suggests that RCV increases turn-out. [But it’s important that voters be given the choice to rank as many or few candidates as they like, so that a voter always has the option of voting for a single candidate (in effect, he would be ranking all other candidates as tied for second). In practice, most voters choose to rank two or three candidates, but a significant fraction just rank one]

Are there empirical studies on turnout.

I believe there are, but don’t know how careful they are. I will try to get you references.

More than u may want to know