I'm not sure about NSYNC holding Justin back creatively.... Afterall, every one of the singles released from "Celebrity" had Justin stamped on it somewhere.
True, but the music he has made since then is quite different from the songs he wrote back then and far more mature, in my opinion.

Besides, true or not, if he felt being part of a group rather than being solo was holding him back creatively, staying in the group would be bad for everyone.