Well, I'm gonna take this into a realistic view. Sephiroth can be considered almost God-like for many reasons. Sora... doesn't have as much as much credability. Although, considering the gameplay moves he has, as well as reaction commands that are especially found in the last world and boss, it's safe to assume that Sora's pretty up there. Now, Final form he's silver, has two keyblades floating around him, and can glide. More realisticly he can fly and can almost be considered God-like in this form. However the fact that Sephiroth merely swept some dirt off his shoulder after fighting Sora can be a decisive arguement. But, it's safe to assume that Final Form can pretty much be comparible to Sephiroth in strength and power. Considering all the facts and assumptions, I believe that Final Form Sora can beat Sephiroth. It'll be hard still, but he can do it.