He has to be stronger than Luffy atm since he's the older brother, like Ace was when he was introduced. I would say if Ace didn't die, Sabo would be around his level, which should be good deal stronger than Luffy. Probably the second strongest in the island after Fujitora.
He should be ace level or higher. He and ace were equals when young, and sabo has had more years to grow truly stronger than ace. Also he has had the timeskip as well.