Well, considering the Majin form is immune to all forms of damage and can destroy everything in a couple of hits, I'd say that power could pretty much exceed Sparda, and if not, is at least equal.
By DMC4, it is said that Dante is more powerful than Sparda, although this is only a rumour, it's only "Said" that he's more powerful, there's no confirmation, but I wouldn't be surprised if Dante's power is greater or equal to Sparda's...At least, when he still had his powers, but even then we're not sure if he's dead, a demon's power would increase with time, so if Sparda IS still alive then his power will exceed Dante's.
I've just confused myself now. :lol: