Who knows how Nero would've fared against the non-corrupt Savior at full power, but I doubt it would've been pretty. Nero got captured by the Savior very easily too.
IIRC, DMC3 Dante might've been able to beat Arkham without Vergil's help... and Arkham had some of Sparda's power. If we assume Sanctus is roughly on the same level as Arkham since he also has some of Sparda's (spoiled?) power, and Nero is able to beat him fairly easily, then I think we can assume that Nero is slightly stronger than DMC3 Dante at the end of DMC4.
He's clearly far weaker than DMC4 Dante, though (''you look as if you've been playing me from the beginning'') and there's probably still a drastic difference between the two at the end of DMC4. So far we've not gotten any indication that Dante is much more powerful than he was in DMC1, and DMC4 only takes place a few years after DMC1, supposedly without any world-endingly powerful types of bosses in between.
So, stronger than DMC3 Dante, but still far weaker than DMC4 Dante, and weaker than DMC1 Dante. How much weaker than DMC1 Dante is still the question, because Nero does get help from Vergil's soul. So for all we know, DMC3 Vergil's experience combines with Nero's.