Quick hit: The only real "DPS" gear you should consider for a DK tank is as follows:
1) Weapon. If it's a 2H weapon and the weapon has mastery on it.... gtg. Especially if its primary stats are better.
2) Trinkets. If you're having little issue with survival on a fight, but the group as a whole could use more dps.... consider a swap out on a trink.
3) Rings. If the ring has a buttload of mastery, strength, and stam compared to what you're currently equipped with, run with it
4) Any other gear that is a significant step forward with mastery/strength/and stam compared to your current. Say.... you're a brand new 85 and you're tanking, your shoulders are the crappy-ass 346 set from heroics..... and you get a fat juicy set of t13 shoulders or t12 equivalent shoulders with mastery/expertise.... wear them.
Hell, even if it's not an ideal pairing, if the new set has enough of a secondary stat that you can reforge to mastery..... or keep the mastery and reforge say crit to dodge..... use them. The stat increase between say, t11 and t13 is such a big jump, you almost have no reason to NOT jump up and reforge.
The main idea is this: if the gear gives a significant performance gain, and can be reforged to be competitive or to minimize the performance loss on mastery/avoidance. you might want to try it.
No one tanks in a void.........