This is a little info I thought I would throw into my site. When Cell self destructed and regenerated, he told Goku and the others that since he had Saiyan(Saiya-jin) DNA, the when regenerating, or rehealing, that he becomes stronger. Thus bringing up Super(Ulra) Perfect Cell with a powerlevel of 300,000,000, but only to loose to SSJ2 Gohan who also had a powerlevel of 300,000,000.