I was fortunate enough to go into Wrath of the Lich King with 5 pieces of Tier 6 gear, all of which lasted me right up through level 80. I'm starting to see upgrades now, but the upgrades mean breaking the%26#0160;amazing T6 set bonus. Is it worth it?
2 pieces: Reduces the energy cost of your Mangle ability in Cat Form by 5 and increases the threat generated by your Mangle ability in Bear Form by 15%.
4 pieces: Increases the damage dealt by your Rip, Swipe, and Ferocious Bite abilities by 15%.
I think it's safe to say the T6 set bonuses aren't very important or bear tanking. Our threat is so enormous we don't really need the extra help. There may be a modest exception for Swipe AoE tanking, but I'm doing OK without the 30% Swipe bonus from talents so I'm guessing the 15% from the T6 isn't crucial. Upgrade away!
The story is a lot more complex for cats. Toskk's calculator is a bit confusing in how it reports set bonuses, so I worked it out in simplicity with my reference settings. Here's the standalone incremental value of the T6 set bonuses as calculated in Toskk's 4.2.3 model:
2/8 T6: 110 kittypoints (+1.5% dps)
4/8 T6: 320 kittypoints (+4% dps)
2/8 T7: 160 kittypoints (+2% dps)
So breaking the 4 piece T6 bonus costs you 320 kittypoints. For the shoulders alone it's not worth it. The stats on the T7 Naxx/10 Shoulders make it 73 kittypoints better than the T6. A nice upgrade, but not worth losing the set bonus. The T7 Naxx/25 Shoulders are a 110 point upgrade over T6: again, not worth breaking the set bonus for by itself. It's probably worth it when you get a second piece of T7, at that point the 2 piece T7 bonus along with the better stats will make up for the loss of the T6.
Note the total value of T6 is 110+320 = 430 kitty points above and beyond the value of the stats on the gear itself. And that bonus actually scales up nicely as your stats improve since it's all percentage increases to damage and saving you energy. You may want to keep that T6 around for a long time.
A word of warning: all this theorycrafting is just starting, no one really knows how feral DPS exactly works yet. Every time I use the Toskk calculator it's been updated and I get different gear weightings, +/- 10%. Also some things are still not fully figured out, like proc cooldowns. Use these rankings as broad guides, not 1% fine-tuning.