Well, since Quickening makes it a free action, and free actions take up no time, then, yeah, you could fire off a Quickened spell and a Doublecast. And Haste would allow you to Doublecast still, too...add onto that a Quickened spell, and you've got four spells in that round....
Which is a lot, but I don't think is really that extreme. I mean, you are still drained after that, right? And it gives the mage a chance to overcome high SR creatures by odds alone...because if they can't hurt something with SR, a black mage is next-to-useless. It gives them big damage potential (five fireballs...yipe), but they still have to be able to use the spells (you can't cast five fireballs until you can kill something by just waving your hand around, anyway)....so I think it would just basically mean that they launch off spells a bit quicker, pile all their eggs in one basket. And if that basket happens to be anti-magiked at the time...heh.
The SR comment comes in when the black mage would fight a monster with high SR. Generally, these creatures are quite powerful already, and the high SR just makes them tougher to hit with any kind of spells. Simply by giving the mage more chances at once, they've got a bit of a better chance of overcoming some of that. It's kinda like re-rolling the check if you fail, by casting another spell. They'll have the same chance to overcome it as a wizard of similar level, but the wizard has a few more rescourses (grease the walkway, escape by teleporting, etc.)
But that's just a minor thing, really. Black mages are *supposed* to be hindered by SR.
I'd be happy to be proven wrong, about the above, of course. Especially doublecast. I *really* want to make that a viable option.
As for Focus...I want it to be similar in power to a feat of some sort...kinda a counterpart to the extra weapon prof. of the sorcerer or the bonus scribe scroll feat of the wizard.
In that light, even + black magician level is a bit potent, though. Perhaps just a flat +2 bonus? (never that great, but always worth doin', sorta level)
I think that actually makes the best sense. Just a flat +2. Makes it about as useful as Scribe Scroll. ^_^
Doublecast I want to balance, definately though. I'm not sure the number of spells in one round is that huge of a factor, but I'd be perfectly happy if I were wrong in that.

As long as I can get something similar out there...something that says "two-spells-at-once" without being overpowered.