my initial thoughts were "it's too hard to judge this as overpowered or not based on what you wrote here. stuff like range, weapon/implement, what defense it attacks, whether the mod damage is from a primary or secondary stat, what class/role it's for, since it's 11th level i'm guessint it's a paragon power so you have to also look at the other paragon features/powers since they all kind of go with each other, etc. all play a factor for comparison"
Now that i can see the rest of the write up, I don't think it's overpowered mainly because it's a paragon path and it seems inline with other striker stuff.
The secondary hit requires a second attack roll so it isn't auto damage.
As for the daze effect, if the creature has any sort of ranged attack it can still use its one action to attack, or if it's adjacent to an enemy it can use a melee attack (or charge). Or it could simply move to a better defensible position or use a heal ability on itself, etc. Daze isn't nearly as bad as stun is.
So, no, I don't see it as overpowered for a striker paragon path encounter ability.