User blog comment:B214/Volume 22 spoilers/@comment-64.237.231.92-20160722012904/@comment-24.113.252.213-20160722171603

Ophis nerfed Issei's DxD G so that he could use it to fight Apophis without suffering the side-affects (intense pain, coma, can't see/hear/touch breasts).

Pre-Nerf DxD G is likely close (I repeat, "close") to Full Power Ophis, or at least Shiva's level (most likely a little lower). I say this because Issei wiped the floor with Rizevim incredibly easily. Although Rizevim is the weakest of the Super Devils, and is weaker than Azi Dahaka and Apophis (who are Heavenly Dragon-Class), he is still a Super Devil, and is likely in the God-Class territory in terms of power.

Therefore, Issei's nerfed DxD G is Heavenly Dragon-Class, but with training and experience, it will reach God-Class (Shiva), or even Dragon God-Class (Ophis).