I've used ACE and X10 arrows for years for target shooting. Never hunted with them though. The cost and availability of outserts for broad heads makes them very impractical.
Way back in the day I shot the small diameter Beman carbon arrows with outserts for the bh. They were pretty amazing arrows for the time. They would penetrate into quakie trees about 2 or 3" and you'd lose your bh in them unless you cut it out. My 2216 aluminums would never penetrate past the end of the bh. I always attributed this, not to the diameter, but to the frequency they vibrated when shot with fingers off a compound bow. The carbon arrows are lighter and stop vibrating much sooner putting all the energy of the whole arrow into the bh. This is almost irrelevant when shooting a release.
As for penetration on foam, the smaller diameter drags way less and you'll see significantly higher penetration with a small arrow and an outsert for bh's vs a traditional .246" id carbon arrow. It is not the same on critters though.
On a side note, the smallest diameter arrows made are closer to 6mm. An arrow can't be made 4mm and be a 400 spine with any material even if it were solid bar.
In the end of the day I don't think it's worth the cost or the stress worrying about such expensive arrows