User blog comment:Kuopiofi/Saitama - Absolute or Supernatural?/@comment-25200606-20161021032403/@comment-27775853-20170522134043

Apparently, it's confirmed in the databook that Boros is star-level, so Saitama overpowering Boros' "Collapsing Star Roar Cannon" puts him at Multi-Star Level in strength (at least I know why Boros' final attack is named like that, it can destroy a star).