I originally posted this on the official Damage Dealing forums, hoping it might catch Blizzard's eye. I'm also posting it here for discussion and hopefully some more math on the actual dps increase granted by this bonus.
Definitions: Tier 12 Mage 4-Piece:
Arcane Power with Tier 12 4-Piece:
When activated, you deal 20% more spell damage but spells cost 10% less mana to cast. This effect lasts 15 sec.
This bonus appears to be a great boon to Arcane Mages' mana pools during Arcane Power. Upon further inspection, however, the math of both Arcane Power and this bonus are misleading. I'd like to shed light on this issue for both Blizzard and the community.
Base Arcane Blast Mana cost: 870
10% savings of 870 = 87 Mana cost reduction
This is 87 Mana cost reduction for every Arcane Blast, regardless of stack number! It is applied after stacking mana cost from Arcane Blast debuff. Arcane Power works the same way, but increases spell costs instead of decreasing them, like the set bonus does. The net effect of the set bonus is two of these mana cost reductions: 1) 10% mana reduction and 2) removal of the 10% mana increase from Arcane Power.
Total Mana savings with set bonus = 87 * 2 = 174 Mana cost reduction
This can also be seen upon inspection of actual mana costs under different conditions in the table below:
AB Stacks Base Cost Cost w/ AP Cost w/ AP and 4pc Mana Savings
0 870 957 783 174
1 2175 2262 2088 174
2 3480 3567 3393 174
3 4785 4872 4698 174
4 6090 6177 6003 174
Suppose 12 Arcane Blasts are cast during the 15 second duration of Arcane Power:
174 Mana * 12 = 2088 Mana saved during each Arcane Power
Let's convert this to MP5 for an easier way to visualize the bonus:
2088 Mana saved every 2 minutes (typical practical cooldown of Arcane Power) =
2088 Mana / 120 seconds * 5 seconds = 87 Mana per 5 seconds
Following the same calcuations but with Shard of Woe equipped, the result is an even lower 67 MP5.
To put this in perspective, over a 6 minute fight, 87 MP5 yields 6264 mana. This is enough for one extra stacked Arcane Blast, which would correspond to a dps increase below 1%. But since this is obviously a rough estimate, we don’t run out of mana to begin with, and the mana savings is actually spread out over several Arcane Power usages, the actual dps gained is even lower than this.
Another way of looking at it is by comparison to Mage Armor. Glyphed Mage Armor with a mana pool of 100,000 yields 3600 MP5. The 4-piece represents a meager 2.4% of Mage Armor's mana regeneration.
Overall, the tier 12 4-piece bonus is not strong enough and does not provide nearly the dps increase expected from such bonuses or granted by the 4-piece bonuses of other classes, possibly due to oversight in Arcane Blast's Mana cost calculation and Arcane Power interaction.
Tweak the numbers of the bonus to make it non-negligible, such as 50% mana cost reduction (yielding 522 Mana savings per Arcane Blast).
Change the mechanics of the bonus to affect base Arcane Blast mana cost so the mana savings propagates through the Arcane Blast debuff mana cost multiplier to provide intended mana savings.
Add another aspect of the bonus to make the entire bonus enticing without affecting the underwhelming mana cost reduction.
Revamp the entire bonus.
If you actually took the time to READ how arcane power works - You'd realize it will reduce the mana cost of ALL spells, not base mana cost as you seem to imply. Thus a max stack arcane blast, will indeed be reduced by 10% of it's cost.
Unless you know, the tooltip for arcane power is broken which for all apparent purposes it is not.
I'd like to shed light on the stupidity and incorrectness of your post, which is typical to typical <Blood legion> morons living in their own little world where not being US #1 is being US #1.