I know there’s been some people claiming that Languish is not that much of an upgrade from 4pc T9. The following is a screenshot of a 10man Saurfang kill I did right after getting 4pc, and you can see that it clearly is an upgrade.
Let’s assume a 60% crit rate on Moonfire, which I think is actually a little on the high side, even given [spell]Improved Moonfire[/spell]. Moonfire did 127973 damage. If 60 percent of them crit and did double damage, 127973*1.60*1.03(meta gem) = 210899.504 damage.
Now, Languish did 108132 damage. 127973+108132=236105 damage. That’s a 11% boost in damage over 2PC T9, and that’s giving Moonfire a very generous crit rate. Giving a 55 percent crit rate, and 204308 damage, you’re looking at a 14% boost.
I don’t theorycraft often, so feel free to double check my math.