Many have been assuming since the announcement of Invocation that the optimal Arcane playstyle will be to spam Arcane Blast until at sufficiently low mana to evocate reasonably, recharge, and repeat. I don't think the numbers make this nearly so clear as seems to be the popular opinion on the official forums.
This post is not meant to be an in-depth analysis with answers about the best MoP Arcane rotation; it's simply a look at some of the numbers involved intended to challenge these assumptions and start a discussion. Most of the information below was posted on the forums, but is collected here in a more readable form.
The Talent Buffs
Rune of Power is simple: a 1.15 multiplier, and an increase in regen/sec from 1% to 2%. Of course, it gets a lot less simple if you have to move, but that's something we can explore in discussion.
Invocation is trickier. The total benefit depends on how often you use it. Optimal is once every 46 seconds — 6 seconds of evocating followed by 40 seconds of casting. Deviating from this in either direction reduces the benefit.
The average multiplier is: 1.3 * time between evocations / (time between evocations + 6). Some examples:
- 40 seconds: 1.3 * 40 / 46 = 1.1304
- 30 seconds: 1.3 * 30 / 36 = 1.0833
- 20 seconds: 1.3 * 20 / 26 = 1.0
With zero haste, net mana consumption per cast with Invocation including the 1% regen over the 2 second cast time) is:
- 0 stacks: 0.5%
- 1 stack: 2.375%
- 2 stacks: 4.25%
- 3 stacks: 6.125%
- 4 stacks: 8%
With rune of power:
- 0 stacks: 0%
- 1 stack: 0%
- 2 stacks: 1.25%
- 3 stacks: 3.125%
- 4 stacks: 5%
And there's the rub for the Invocation Spam concept
At four stacks, you're losing 8% mana with every cast. You'll need to evocate every 16-25 seconds (presumably spending some time on Arcane Missiles), effectively negating your Invocation damage buff. A four-stack spell only does about 14% more damage than a three-stack spell (200 / 175 = 14.29); factor in the lower average Mana Adept bonus, and it doesn't look to me like you come out ahead.
Here are some examples if you want to play around with the numbers. Since you're comparing multipliers, you don't really need to worry about base damage. You can just compare the multipliers. Higher multiplier means higher damage.
Mastery Multiplier * Mana Level * Arcane Charge Multiplier * Buff Multiplier
- Base mastery, 92% mana four charges, 20 sec evo: 1.12 * 0.92 * 2.0 * 1.0 = 2.0608
- Base mastery, 76% mana, four charges, 20 sec evo: 1.12 * 0.76 * 2.0 * 1.0 = 1.7024
- Base mastery, 93.875% mana, three charges, rune: 1.12 * 0.93875 * 1.75 * 1.15 = 2.1159