Depends how you define successful, but I always figured a Taiping that "won" would fall into internal power struggles pretty quickly, given Hong Xiuquan was incapable of providing effective leadership (he preferred hanging out in his palace having visions, interpreting scripture, and beating his wives). Even IOTL we saw some internecine feuding amongst the Taiping leadership. Once Xiuquan dies, I doubt Tianguifu can hold things together (even if he has someone like Rengan to assist him)- you'd either get Taiping generals doing their own thing or competing to dominate the Heavenly King.
In any case, with the widespread devastation, possibly prolonged unrest in the countryside (peasants who don't like have their traditional idols smashed, gentry spooked by the Taiping's rhetoric about land reform, soldiers going feral), trying to establish some workable diplomatic relations with the westerners, and possibly having to deal with the other ongoing revolts in the east (Miao, Nian), I doubt the Taiping would be in any position for radical expansion for a while.
Also, hadn't the Qing signed Manchuria over to Russia in 1858/1860? I'm not sure the fledgling Heavenly Kingdom can afford to get into a confrontation there.