There is a good on-line article by John Barsness, a gunwriter who is knowledgable and tests his premises, on the 9-twist .223s. A lot of rifles are going that way with the heavier bullets. It's on the 24-hour campfire website. Good reading.
He says there is no problem with the faster twist. I guess that's true as I've shot some pretty good groups with a AR with fast twists. It certainly adds versitility to a .223 with heavier bullets.
Folks like to use the term "over-stabalize" but that's not a term that has meaning, as a bullet (or anything) is stable or not-stable, but not over stabale. I'm doubtful you can spin a bullet too fast in .223 Rem so the stability is affected adversely. Maybe with a 22-250 or a .220 Swift, maybe not even then. Bullets can come apart if there is a problem with the jacket on very frangible bullets; never seen it on a .224 bullet, but have on a .17 Remington.
I've got a CZ that I think has a 12-twist in addition to the M 4 9 twist, and every thing I've done informally bear out Barsness's tests. All shoot 55 gr. bullets well enough. The most accurate AR I've owned had 1:7, IIRC, and shot 69 gr. bullets into bugholes. I doubt the CZ would stabalize the heavy bullets as well.
I think it's better to err on the side of faster twist, and it does open up the possibility of heavy, longer-range bullets for longer range shots with high BC possibilities.
A 12 twist rotates a bullet once every 12", or 300 times in 100 yards. A 9 twist turns it faster, do the math, 400 times and a considerable jump in percentages. I don't know if this matters, but it matters the other way, as you need faster twists to turn longer and thus generally higher BC bullets to make them stable. At longer ranges this is especially true.