User blog comment:B214/High School DxD EX/@comment-187.209.37.200-20160123193708/@comment-26821288-20160123212151

Wasn't stated.

Although, if he reached to a Super-Devil level of raw power in about a year after he was reincarnated as a Devil, then 29 years after it......he would be some asses-kicker, in my opinion, and would probably have a class of his own, just like Ophis and Great Red. Although, the author said that Issei's power-up in Volume 20 would be his last power-up (or so I heard), so he maybe won't go as far as having a class of his own... But, Issei did say, in Volume 20, that he wants to become the strongest dragon, and he does have room for improvement in his power-up, so... He might actually become something beyond our expectations.