Why didn't the First and Second Hokage's fight with full power against the Third?
Revived or not, they're still not the real deal. The Third was not fighting against the real Shodai and Nidaime. That would have been perhaps a completely different fight. 

Besides, the fact that Orochimaru's Edo Tensei might not have been that perfect and powerful (perhaps less perfect than Kabuto's), makes me (and not just me) think he used it for the sentimental effect it would have had on the Third, i.e. fighting against your predecessors, rather than actual relying on their (weakened) power only.