So, are haste tresholds really a DPS increase(cost of mastery) if you 90% of time reapply a DOT before last tick? For me Unstable Affliction is propably only DOT i refresh when its on its last tick. UA haste breakpoints are 872 and 6637. The usual scenario is that i have INT proc when UA is finishing and there is 8-12 secs left on corruption and then i refresh all with SB: SW if i have more than one soul shard, otherwise manually and haunt.
Reaching a breakpoint means that that dot's duration goes back up to its full duration. Instead of a 15 second dot it's now a 20 second dot, and over the course of a minute you have to refresh it one less time. (3 times vs 4). That's more GCDs spend doing other things and thus more damage.
That being said, people over-estimate the gain they get as affliction in reaching haste breakpoints. (I'm not saying they aren't an increase, I'm just saying that stacking mastery after reaching one or sacrificing mastery to get one probably isn't the way to go)