Tier 12 2-piece bonus and VPLC increases crit
A lot of people in 4.3 are still stuck using tier 12 gear. For these people, crit will be simming pretty close to haste, and very often higher than haste. Although they very close, you do not want to stack crit. However, you do want to consider crit better than mastery in this situation. So if you're still inbetween tier 12 and tier 13, your stats would be something like this:
Int 1.0000 SP 0.7589 Has 0.5361 Spi/Hit/Crit 0.5134 Mas 0.4792
That should hold you until you drop the 2 piece bonus and VPLC.
Tier 13 4-piece bonus increases mastery
The 4-piece set bonus increases the value of mastery by about .05. The reason for this is that it has a large impact on Mind Blast damage - more specifically, the majority of your mind blasts will be with 3 orbs.
Looking at a log without the 4 piece bonus, Mind Blast types are spread pretty evently:
MB0 - 6, MB1 - 13, MB2 - 14, MB3 - 17.
After turning the 4p set bonus on, it skews highly in favor of MB3:
MB0 - 3, MB1 - 4, MB2 - 5, MB3 - 38
This is the explanation for why mastery scales so much higher with the 4p set bonus.
For stat weights, you could consider the following weights to be "practical":
Int 1.0000 SP 0.765 Has 0.5539 Spi/Hit/Mas 0.548 Crit 0.5387
However, how closely simulationcraft follows these stat weights depends on the weapon you are using, and whether or not you have DI.
1. Without the legendary staff, and with Dark Intent active, you don't seem to need much more than about 30% haste, or in other words 2141 haste rating. So with the 4p set bonus, no staff, and DI, you can trade some haste for mastery after reaching this point, and simcraft will sim at a higher dps. You should get more DPS with a mastery-heavy build under 3202 haste than above it.
2. In any other situation, you will get more DPS if you stack haste past 3202, and then mastery. So you do not need to make any changes, and simcraft should represent a solid dps increase by sticking to the stat weights I've listed.
What do we do?
The difference it seems between stacking mastery rather than reaching 3202 (without a legendary, with DI) is about 600 dps. So if you find yourself in situation #1, ask yourself if this amount of "potential" dps is worth it. If the answer is yes, play around with swapping some haste for mastery. See if you notice a difference. Otherwise, I would probably just leave it alone.
I hope that people find this information useful.