He will never be one of the strongest captains during the series. It would take centuries after the end series, but then he'd be one of the most OP Shinigsmi in history considering as a child he's captain level. Basically once Toshiro would be at an age like Byakuya's... He'd be stronger than Byakuya at that time (assuming his potential was maximized).
Toshiro right now could possibly fight with Soi-Fon and Komomura. The other Captains are stronger. Ichigo doesn't even need to go Bankai to beat Toshiro. ( ._.)