9/10
I really like the way you gem, and more so --- I really like you matching the sockets -- that is a rapidly dying style (for good reasons... I suppose)
Replace the helm with dark conjurer's helm, and replace the brutal ring with Band of the Eternal Sage, and the ring of omipotence with Loop of Forged Power (IF you can afford it)
In terms of breakpoints as a mage.....
You definately want 25%, so when you pop your icy veins and than frozen orb, they are ticking as fast as they can (50% haste is the GCD cap, so after 50% global cooldowns and moves like frozen orb dont benefit anymore)
But to be honest you don't have to have exactly 25%, because if you pop icy veins / skull or gul'dan (or haste if you are engi) than summon mirrors, they really kick some ass at lvl 70 and just cast so fast, mirrors can benefit from haste up to 100%.
As for the skull vs power circle... I've tried to make the power circle work, and it CAN work, in BGs SOMETIMEs... But SOOOO Rarely do you get to stand still, and good players in this bracket KNOW that power circle, and will retreat at the 1st sign of one. You have no idea how many times I see someone pop a power circle, than run out...Makes me laugh because it's a huge waste....
Engi vs JC is tough...
If you are going to do tons of arenas, go JC imo because those stam gems are nice, but if you love random bgs, you HAVE to go with engi for the nitro boosts.
I like your setup, sorry if I rambled, dont feel like proof reading this or anything xD
You can check out my mage if you want to get some ideas (i have a million sets, so you never know what you'll get =P)
Valdius @ Moon Guard - Community - World of Warcraft
TLDR --- 9/10 --- 10/10 when you get 2nd prof, band of eternal sage / loop of forged power / dark conjurer's cowl.