I've shot a .357Mag indoors with a full-power self-defense round. It didn't stun me, but it did leave me totally deaf for a few minutes and badly impaired (very hard of hearing and with significant ringing) the rest of the night. It improved over the next few days and the ringing rarely bothers me anymore, but I never got all the hearing back in my left ear.
That's from ONE shot.
If you have to shoot without hearing protection in a do or die situation then do what you have to do. If you don't have to shoot without hearing protection then don't do it. Whatever small benefit you might gain from practicing without the proper protective equipment is not worth the damage you will do to your hearing.