@Chomps:
1. This video is virtually useless in diagnosing your grip. We need to see a closer video from the support side of the gun.
2. If you're shooting 250 rounds straight just doing fairly slow splits shots on a target, I'm not surprised your hands are tired. I never practice this way.
3. Above the hand-joint, my thumbs have nothing to do with the force of my grip. This does not seem like something to worry about. Maybe just take your thumbs away from the gun and focus on the rest of your grip?
4. At the speed you're shooting splits, your group doesn't tell us much about your grip, and your grip may not have much to do with your group size. A good grip allows the gun to return consistently, so the sights remain on target. Shooting a 2sec Bill Drill, or the Doubles Drill, will diagnose that.