It's a bit tough to predict his box office power here.
On one hand, his last 2 Broadway shows recouped - which is actually a pretty stellar track record, these days. But An Act of God was at the height of his Big Bang Theory fame, and in Boys in the Band, he had some other stars alongside him - it was more of an "event" piece, rather than a "come see Jim Parsons" piece. So it's tough to know how many tickets he'll sell by himself, now that now that Big Bang Theory is irrelevant.
We've seen before that a tv/movie star in a musical often sells better than a tv/movie star in a play. But then again, this intimate, lesser-known musical might as well be a play, for all its marketability and broad-appeal (I love the musical, personally).
On the plus side, with John Doyle's staging, it'll be capitalized at approximately...what? A thousand dollars?
As for whether it's a Tony-worthy performance - I thought he was awful, but I saw it in early previews, and I heard he improved somewhat. So take that how you will.