The only cap is the amount of +skills you can use, which is lots with the right equipment, but it's not a real cap anyway.
The reason it still says 11-11k is that the charms didn't add enough damage to make it closer to 12k than 11, thus it still shows whatever is the closest thousand.
Offensive ones don't add that much to your hammers, if they were combats instead you'd probably hit 12k, or even 13..I don't remember the exact numbers.
You can see your real damage by hovering over the skill in the skilltree, or any place that isn't the characterscreen.
It does in char screen only (as far as I know)
Try to switch concentration aura on/off and you'll notice that the dmg shown in the character panel changes (it take a second to refresh). It works wether you are in town or not.
I allways check this when playing with another hammerdin, and if my dmg are better, it means that his concentration lvl is higher than mine so I switch to another aura (meditation usually).