this math is right, however, the number you get are quite different in real fight. i means, more higher the stats will be, more the effect of the increase of 10% will be important. and the magical damage of the black mage are already quite high, indeed the trouble of the immobility of the cast it's to take in account, but actually, in real fight... no one can beat a blm dps on fight of 1-2 minutes.As to people wondering about the DPS increase, here's some math.
There's a simple equation you can use to determine BLM pot/GCD (explanation here; if I got any of the numbers/casts phase wrong, feel free to correct me). Currently (2.1), BLM has...
(275 + .25 * 340) + (220 * .5 * .8333) + ((150 + 70 * .4) * 1.8 * 6) + (220 * .5 * .8333) = 2465.73 potency over 9 "GCDs" = 273.97 magic potency per GCD
With the potency increases, BLM has...
(275 + .25 * 340) + (240 * .5 * .8333) + ((170 + 70 * .4) * 1.8 * 6) + (240 * .5 * .8333) = 2698.39 potency over 9 "GCDs" = 299.82 magic potency per GCD
This means that BLM will see about a 9.4% (299.82 / 273.97 - 1) increase in total damage, which might as well just be a 10% increase.
like i did said, the black mage is fine in BC2 now, but in the rest of the content, like dungeon... it's not the case, this increase can be seen and feel quite easily. since the fight are shorter the damage output can be quite impressive and i'm concerned that this boost that was only for 4 raid will have a real bad impact on the whole balance of the game. the balance of a class must'nt only based on the endgame that not the most important part of the game.
ps: i had experence of blm in dungeon simply chaining fire 1 of 1000-1300 (without critical, nor fire 3), it's impossible for any other class to reach this sort of dps in the soo little time...