User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170825182223/@comment-31137559-20170827021245

I really don't think Issei being able to teleport through magic circles meant that his demonic power increased, He probably gained more control over the little demonic power he had and learned how to utilize it better, but I really don't believe his demonic power increased. That's just his base demonic powers. I'm not including how much demonic power Issei has after boosting or how much demonic power he has while in the Boosted Gear Scalemail.

And plus what training to increase demonic power? If it was possible to increase demonic power by training, Sairaorg would have done that by now.