Well on single target encounters my DPS seldom reaches 18k dps. Well most encounters have lots of running around, that hurts them more then me. (We are quite caster heavy.) However I often do more than 4k dps in raids. My gear is only a little bit better than your's I think.
One thing you did not answer in your post: Do you use HS on CD? You should do so, because with some vengeance it's better than every other ability we have. You also should think about using Inner Rage. Either makroed to other abilities or manually what ever suits you best. In many situations you should have enough rage even when it is up.
I took a look at your spec. I think you can improve it dmg/threat-wise. Incense would be a big boost at least before the next patch, but I think it stays good with the Inner Rage changes, even as HS itself gets nerfed. Alternatively you can go for War Academy and Deep Wounds. Anyway, I don't know why you took Booming Voice. Sure 5 additional rage may be nice at the beginnig. But only then. We are flowing in rage later on. So why reduce your dmg and threat output (or your utility) for 5 rage every encounter? Even while I have it myself, I don't know if you need Piercing Howl in a raid-spec. If you are on kite duty, it may be nice to have, but I don't know about your tanking assignments.
Blood Craze is a weak talent, too. As you can read over in Theory and Mechanics Discussions. (To lazy to link thread.) But if you want to have Piercing Howl it's ok to take.
All those changes to your spec would probably change more than few additional points into hit and expertise - without hurting your survivability. You will probably be happy to have all this survivability for some encounters in Blackwing Decent or Bastion of Twilight.
Your dps output will vary widely from encounter to encounter. Sometimes you will be quite high and fighting with your best dps. Sometimes you will be much lower. (My raid tells me I'm clearly a DD anyway, while looking at my dps output, when I complain about having to spec to my dps-spec because the other tank has only a healing offspec and no gear for it. But there are other encoutners where I'm also around 4k. First 3 bosses in TotfW for example.)