User blog comment:B214/Volume 21 info & spoilers/@comment-191.82.149.185-20160706043402/@comment-24.113.252.213-20160706180636

In a [Full Powered] DxD G mode, Issei has Ophis' power of "Infinity" + Ddraig's power.

Vali's [Full Powered] DxD L mode has his "Lucifer" power (which is Super Devil Class at best) + Albion's power.

Without the nerf, Issei is definitely way stronger Vali.

But with the nerf, considering they both defeated Heavenly Dragon-Class beings (Apophis and Azi Dahaka), I'd say that Issei and Vali are about equal in strength.

But Ophis made it so that Issei can make the nerfed DxD G mode stronger by training his body (which is made of Great Red's flesh) so that he can handle Ophis' power of "Infinity" that comes with it.

In other words, for now, Issei and Vali are equal. But with training, Issei will become stronger than Vali.