A guy who throws for 3000+ yards in consecutive seasons is an above average QB. However, that does not always translate to victories, when he also throws a whole lot of INTs.
I don't think comparing his potential success to his failure in Buffalo would be a good comparison. These are two different teams with different type of coaching staff.
Why is Pettine's defense relevant here. Fitz failed to rally the team and would thrown game ending INTs at inopportune times.