Yep, I use the Captains of Crush grippers 2 - 3 times a week. Before my last neck injury that affected my left arm/hand I could easily close the #2.5 with both hands. After the neck injury I can't close the #2.5 any more with my left hand, but I can close a #2. I am still working on getting my left hand grip strength back to what it was, but I think the permanent nerve damage to my left arm/hand will keep me from getting back to 100%. My right hand is still the same strength as before. I can almost close a #3 with my right hand.
From watching your slow motion videos its hard to tell if the excessive muzzle flip is due to actual grip strength, friction on gun, or wrist angles. If you take a slow motion video of the support hand side that will tell more of the story about your grip on the gun. If your support hand is staying stationary and the grip is moving within your hand then it could be a grip strength or friction on grip issue. If your support hand wrist is bending along with the movement of the gun then its usually an issue with your wrist not being locked forward in the canted position properly.
Do you lock the wrist of your strong hand as well?