I'm having my doubts about Sparda's 'power'.
Mainly because apart from failing to destroy Mundus and the hordes of Devil Generals inbetween them, we haven't heard anything about him.
So far, Dante has surpassed his father and defeated Demonic Foes whom his father left alive, Phantom, Nevan, Griffon etc. So, from where I'm standing I feel that Dante has gone beyond the bar his father has set, Vergil being close behind and then Sparda. Nero trailing somewhere behind all of them.
Afterall, if Sparda TRULY has/had double/triple Dantes power/strength, then why was Phantom left living? Why didn't he use this to actually destroy the Prince of Darkness; Mundus? Hm.