Yes, I agree that strike to stun is powerful save to enemies who have thick metal helmet on their head. Often it is a must have for a melee fighter, and plenty of classes have access to it.
It has some major restrictions, like you should use weapons with Pummel quality. This may be a drawback when ...

As far as I concern, it will significantly change the difficulty of the game.
Try to get used to SL rule, yet I highly recommend using alternative advantage rule from Up in Arms.