Well, i go to karazhan a lot as a fury spec, but then again im just lucky. Spent all my money basically gearing myself up, and i make a very comfortable second place on the dps meter, second only to our guild master (rogue). With better gear i could maybe out dps him. As long as i have blessing of salvation on at all times, i can go full dps and not pull agro.
Whether or not you'll be able to dps or not in end game is partly down to luck, and how open minded your guild master is towards "off specs" such as shadow priests.
I didn't even originally want to dps, but we have a very good prot warrior and feral druid, so i wasnt needed as a tank in my guild. Works out for me, means i can quest and pvp and raid with the same spec.
Basically, you might be lucky enough to dps, but don't be afraid to tank. I find tanking fun anyway, its nice to feel needed :)