User blog comment:B214/Question & Answers 5/@comment-5.36.246.21-20160220200141/@comment-26345952-20160221010144

For now, most of his powers comes from the Boosted Gear, so he is more acoustomed to Ddraig's power more than anything else.

Though I say that given that his body made out of Great Red's flesh and Ophis's power he, in theory, probably does have the potential to reach at least a status close to the heavenly dragons. However he has even mastered that power.

So for now Issei up until vol 20. can only reach a power level that high because of DxD form.