User blog comment:B214/Question and Answers 2/@comment-24098614-20140506084129/@comment-1359322-20140506085721

First of all there is not such thing as magical cores in DxD, for starters the more advance a magic is, the more calculation is needed, that's why Issei can't use any impressive magic. As to his demonic powers, there's none too. He just got a new body that's all. Any improvement whatsoever has yet to be shown.