I shoot a XD40sc and 4". Have put up to 250 rounds down range at a time, with no problem. I don't grip with my thumb, but fingers. My thumbs are parallel and aligned with the slide below the slide lock but not gripping. No numbness or soreness in my hand at all.
I do have a problem with my trigger finger, which was broken many years ago at the first knuckle, resulting in about a 20 degree deviation from normal, toward my middle finger. This resulted in the side of tip of my trigger finger getting a little beat up on the trigger guard under the trigger. A knowledgeable gunsmith and former competition shooter suggested removing some of the material under the trigger, especially at the break point. Not the much material was removed and looks fine, but most importantly it solved the problem.