Arcane Mage T12 4-set...Good? Bad? or Ugly?

I'm going to take my best guess here as to how this T12 4-set bonus "Arcane Power decreases the costs of your damaging spells by 10%" works. If anyone knows differently please correct me.

Currently:

0.050 * 17,418 = 870.90 (ABx0) 5% of base mana
0.125 * 17,418 = 2177.25 (ABx1) Base 5% + 7.5% = 12.5% of base mana
0.200 * 17,418 = 3483.60 (ABx2) Base 5% + 15.0% = 20.0% of base mana
0.275 * 17,418 = 4789.95 (ABx3) Base 5% + 22.5% = 27.5% of base mana
0.350 * 17,418 = 6096.30 (ABx4) Base 5% + 30.0% = 35.0% of base mana

I imagine this means that if Arcane Power is active then:

ABx0 costs 87.1 less mana to cast
ABx1 costs 217.7 less mana to cast.
ABx2 costs 348.4 less mana to cast.
ABx3 costs 479.0 less mana to cast.
ABx4 costs 609.6 less mana to cast.

Currently in a raid environment I have a 1.2 second cast time with everything reforged into Mastery/Hit, so my haste isn't all that high, but that's what the stat weights dictate currently, so that's what I've done. This of course takes into account my T11 4-set bonus of 10% faster arcane blast cast time.

Therefore, in 4.2 I will be casting arcane blasts 10% slower. Let's just guestimate and say in a raid buffed environment, I will now be casting Arcane Blastx4's @ 1.3 seconds instead of 1.2.

That being said, Arcane Power lasts 15 seconds and can be cast every 1 minute and 30 seconds. Provided I wait till I have a 4 stack, and cast nothing but Arcane Blasts and keep my 4 stack the whole time, I could theoretically get off 11 AB's within the time Arcane Power is active.

11 x 609.6 mana = ~6706 mana saved per Arcane Power.

Let's say an encounter lasts 10 minutes.

I can cast Arcane Power *6* times during that encounter, and provided I use it optimally (as stated above) all 6 times..

I will save a grand total of 40,236 mana for the entire 10 minute encounter.

I'll leave it up to you guys...

Do you think this is a worthy T12 4-set bonus for Arcane when you compare it to Fire's 30% chance to proc Hotstreak on crits, and other classes 4-set bonuses?

(This of course is all based on whether or not I'm correct on how the 10% reduction in mana costs works with the 4-set bonus)

Edit: After testing Arcane Power on a test dummy, I discovered something interesting. I noticed that currently (4.1 live), it seems to work off the base mana cost of Arcane Blast, and it increases the cost of our Arcane Blasts with 4 stacks by only 87 mana (870 base AB mana cost x 10% more = 87 more mana to cast).

This kind of concerns me, because if this is the way it works it's not really much of a penalty at all, and I'll have to admit I thought it was a lot more than an 87 mana penalty until I just tested it.

On the flip side, it will probably work the same way with the T12 4-set, but instead of increasing the cost by 87 mana it will decrease it by 87 mana. This would be almost akin to having no 4-set bonus at all if this is the case. 87 mana is nothing when you consider how much stacked arcane blasts cost to cast.

Can anyone shed some light on this?
It's kind of meh but because arcane scales so ridiculously well with gear / int the spec will probably still win out single-target once in heroic modes.
I heard from Lhivera or someone else credible that it only decreases AB mana by the baseline amount, not scaling.

Of course, I could ludicrously off.