@GeneralVan: I'd have to look through volume 14 - 16
Vali is more of a rival rather than villan
Rizevim apparently has more on his plate if Sirzechs made his whole peerage to fight him.
@GeneralVan: The other characters get power ups as well just not as much as Issei, but at certain points their stats become comparable I.e while sparring issei admits kiba is as fast and more agile than him even in balance break, though before triana.
I agree its not DBZ level in power from where I've read up to so far. But I've noticed that dxd is the most training-powerup based franchise since DBZ and yu yu hakusho, more so even than the HST or power 6. Its like DBZ with tits.