How close are CSD and ESD for meta gems? 21 Int will always beat 21 crit, but I find it difficult to think out the secondary benefits, especially given the shifting nature of Mana Adept.
Does that 2% maximum mana provide a decent bonus to our Mana Adept damage, or does the 3% crit damage rise above that as well as the difference between equal amounts of crit and Int.
I think at one point earlier in the thread it was stated that CSD was still better, but with things changing, I was interested to see what the gap between them was now.
Some quick napkin math:
Assuming you have 35,000 mana without ESD, ESD provides an extra 35,000*.02 or 700 mana. Hence, at maximum benefit ESD ensures you have 700 mana more at any stage of the fight than you would without it. For instance, a cast at the 30k/35k mana lavel (or ~85.71%) without it would be a cast at 30.7k/35.7k (or ~85.99%) with it. That's right-- at roughly 85% mana level the ESD provides a very, very paltry 0.22% increase in mana level to apply to the mana adept bonus. The absolute best case (although not very realistic) is a cast done at 0 mana without ESD (for a 0% mana level) would be cast at 700 mana/35.7 (or ~1.96%) with it for a 1.96% increase in mana level. Hence, the ESD provides almost no benefit at high mana levels, and only very marginal benefits at very low mana levels. Even if your mana adept bonus was at 50% (which is generously high), and your average mana level without ESD was 50% (which is generously low), the damage bonus from ESD with mana adept master would be .5 * ((17500 + 700)/(35000 + 700) - .5), or a 0.49% damage bonus.
CSD provides a 3% increase to crit damage, which is 1.5*1.33 times base damage. This means CSD provides roughly a (1.5 * 1.33 * 1.03 - 1*.5*1.33 ) =~ 5.985% * C damage boost, where C is your crit chance. Even at a bare-bones 22% crit chance (obtainable through raid buffs alone), that means CSD provides a bare minimum of a 1.32% overall damage boost.
In other words, it doesn't even appear to be close.
In crunching the numbers, it looks like a (slightly) better question is: is the Isightful Earthsiege Diamond worth considering? It provides +21 int and a 5% chance on spell cast to restore 600 mana. Assuming a 2 second AB cast time, this works out to roughly 75mp5 (EDIT: actually 55mp5 because of IED's 15-second cooldown). This means over a 5-minute fight, you'll get 3272 mana back-- which is far better than starting off with 700 additional mana. Even better, since the IED doesn't affect your maximum mana pool, it doesn't negatively impact your mastery bonus. In fact, over the course a 5-minute fight you can expect your average mana level to be approximately 4.4% higher ( 54.5mana higher for the first 5 seconds, 109 mana for the next 5 seconds, 163.5 mana for the next 5 secons, etc). At 0 mastery rating, this would be a 0.52 % damage bonus, but with 24% mastery bonus this would be a 1.04% damage bonus. Far better than the ESD, but still probably not competitive with CSD.
Obviously in real life you wouldn't achieve an average 4.5% higher mana level-- you would just end up casting more ABs at higher stacks. And the shorter the fight, the less the benefit. For instance, for a 55-second fight the average mana level would only be 0.6% higher, everything else being equal.