User blog comment:B214/Volume 23 discussion blog/@comment-172.58.17.194-20170329205153/@comment-26821288-20170329225659

I would say that in 30 years, yeah, Issei will have power that rivals that of Great Red. I mean, he has Great Red's power sleeping in him currently, has Ophis' infinity with DxD G (which was nerfed, but will likely get back its power, since it was unstable because Ophis is unstable), so combining these two alone would make him probably as strong as Great Red.

Also, he has seemingly complete control over Dragonification in 30 years, which would mean he porbably already mastered Great Red's power and DxD G, and maybe even some more powerful powers, for all we know.