At this point, I'm going completely OT on my own thread, but wasn't sure I wanted to resurrect the other...
I came up with another, possibly more agreeable option, since the concern seems to be that extending the spell wouldn't be so bad if it weren't for the accuracy bonus- ie, extra duration reduces weapon grade neatly, but doesn't reduce bonus damage, and allows for, ultimately, a net damage profit relative to the stress cost.
What if you treat the bonus accuracy damage the same way you treat it for split or zone attacks, and only allow it to apply once, split over several rounds?
IE- If I make a Weapon:5 spell that lasts 4 rounds (8 shifts), and roll an 8 on my control (barely got it), vs their defense of 4, instead of doing 9 damage the first round, and possibly 9 more each round til it runs out... you have to split the 4 bonus accuracy damage up among each round... you can front load it, so round 1 does 9 damage, and each after that only the minimum of 5 (minus armor), or spread it out, and do 6 each round (minus armor).
In the case of such attacks that are ALSO split attacks or zone attacks, it has to be split even further.
In favor: it limits the worst of the extra damage, and uses an already existing mechanic (always a good sign, IMO)... furthermore, it uses that existing mechanic in a way that makes philosophical sense, since you really are just splitting the bonus damage among multiple targets- those targets simply don't occupy the same time-frame. If some of those multiple targets happen to have the same face and name- eh.
For my purposes, for my character (frost sword spell, weapon:5, 4 rounds), I was also planning on him not being able to use any other magic while he did this particular spell- he'd still have to direct it, and so couldn't afford the concentration to double-dip on the action economy. This would merely allow him to pace himself better when it comes to his mental stress by doing a significantly less damage than he's normally capable of, spread out over a couple rounds... and I'd probably only use it in certain circumstances (like when I'm running out of mental stress and still have a ways to go in the current fight... or have a lot of mobs to worry about fighting that I can't safely roast en-masse... endurance fights, essentially).
Similar limitations are being applied on my player as far as not being able to double-dip on the action economy, or stack up on accuracy bonus.