http://elitistjerks.com/f80/
That forum contains threads that will provide you with every single piece of information you could possibly need to know. Pick whichever thread is relevant to your spec. If you cannot understand what is being presented, you should probably stop worrying about your DPS.
---------- Post added 2011-09-15 at 09:57 PM ----------
As a demonology warlock, these are the thresholds listed in the EJ thread. If you can reach these thresholds through reforging, you might see a DPS increase. Going much above or at all below them, however, will lower your DPS.
Mastery is your #1 rating for Demonology.
Code:
Therefore, the haste thresholds are (raid buffed):
Extra Ticks % Rating Goblin
1 7.2% 0 0
2 21.4% 1573 1431
3 35.7% 3264 3105
4 49.9% 4951 4776
Corruption gains additional ticks at the following levels:
Corruption
Extra Ticks % Rating Goblin
1 8.3% 21 0
2 25% 1993 1846
3 41.7% 3970 3804
4 58.4% 5945 5745
You are not a goblin, so you can ignore that column.
In other words, 1993 (or slightly over) is the most haste you should try to achieve. Being under by a single rating makes it pointless. Being over by a little bit is fine. The rest of your gear should be focused on mastery.
And, as stated in the quote, this is assuming full haste raid buffs.