My first reaction to this one was questions: Why only metal helmet, wouldn't a leather or no helmet also hurt? Also, tying the Wisdom save to what I assume is the pain threshold, that doesn't sound quite right, although I see what you're going for, mentally dazed from the attack. Lastly, this ability feels like it's dipping pretty far into 4e character powers, this one seems quite suitable for that. Nothing wrong there, but something like this might be better suited as a houserule for a critical hit: instead of doing Double damage, you do X-Y or Z.
Anyway, wasn't trying to negate your efforts, what are your thoughts?
The effects are ripped right from the SLOW spell, WIS save and all. Changing it to CON is no big deal, works either way IMO.
In my mind, it's nothing more than a six second concussion.
The metal helmet was just a flavor thing. I really like the mechanic of SHOCKING GRASP, maybe that is more appropriate, something metal armor guys have disadvantage on their saves (cause of the ringing!). I actually think this would make a cool magic item, but in my mind this is the perfect ability for an Ogre with a level of fighter or something.