@lime I think that since multiwinner elections are generally for electing legislatures, we can say that generally the goal is for the set of winners to be able to vote on laws in such a way that the laws passed maximize voter utility (i.e. come closest to their preferences on the issues). Now since this is difficult to simulate, how about simplifying it down to an election-based process: have the winners of the multiwinner election conduct a single-winner election among themselves to choose a leader (presumably using whatever single-winner voting method the original multiwinner election's voting method simplifies to), and then see how much utility the original voters in the multiwinner election receive from that leader compared to the amount of utility those original voters would get from the leader they would have elected if they had themselves voted in the single-winner election.
Essentially, test how much utility voters would get from the Prime Minister chosen by Parliament versus their utility if they directly elected a President instead.

The same idea is covered at [https://rangevoting.org/BRmulti.html](link url)