User blog comment:B214/Volume 22 spoilers/@comment-27083298-20161112215056/@comment-26821288-20161114220448

I think it might make Issei stronger if he will train his body like Sairaorg did, until he can already use Touki from training his body. I mean, it would make him stronger. And he will need to get his body trained a lot to actually use Touki, so until then he would be as strong as in his BB without using BG at all. So imagine how strong he would be like that while using CCP or DxD.

Another thing Issei might get stronger through is Dragonification. If Issei will control his Dragonification well enough, he will be, maybe, able to transform into a Dragon and get much stronger.

But, still, combining these two things... Issei will become OP. But I have to say, I wish Issei would train again with Tannin sometime...