I certainly do believe that the Cavs need Irving to stay with Golden State but if I assess this with the assumption that Irving plays about as well as he did in the last Atlanta game (22 min, 16 pts on 11 shots, 4 rebs, 5 assists) for the entire series, I think that will be huge.
The Cavs aren't going to outplay or outscore Curry/CThompson/Livingston with Irving/Smith/Shumpurt/Dellavedova but if you look at James/TThompson/Mozgov, that is quite a bit better than Barnes/Green/Bogut in my opinion. Even just look at Thompson/Mozgov vs. Green/Bogut. I don't see an edge there for GSW, or not enough of an edge to make a difference. I think Green and Bogut have been talked up too much. Green is a nice versatile player but I am not sure he is going to put up much better numbers than TThompson in this serious.
Iguodala does give GSW some depth that Cle can't match with Marion/Jones/etc but again, I don't think that is enough of an edge to decide a series. The most lopsided match up is James over Barnes (considering position, I know they may not actually cover each other but officially, they are their team's respective starting SFs).
The problem for Cle is that Irving will probably not hold up for the whole series even if he gives them a spark to start. Tough one to call (for the record, I am one of the ones that voted for Cle).