Okay, back on topic here...
The CA M249 has 8mm bearings, the reason they say 'Don't use a 9.6 or greater', is because those brearings give you a nice RoF boost. They let the gears spin faster, sending more force to the piston. Considering that CA's yellow piston isn't a great piece to begin with, this isn't a good thing. With the CA M249's, I've heard of lots of guys simply stripping out pistons. Less likely would be a total gearset failure if the factory shimming was bad enough.
Now we start getting into the really technical stuff.
From what I've seen in examinations of CA pistons, they seem to need a slight AoE adjustment as well as a Sector gear clearance adjustment on the second tooth. Why? Hit up this
LINK for a quick explanation.
Once you've got the AoE adjusted, you can really go to town. Get a decent gearset, good shimming, MOSFET trigger, and whatever motor/battery/spring combo you want. If you run a Guarder SP100 (near equivalent to stock CA, but more consistent) you can run it to around 30RPS, at a minimum.
And yes, at that point, people will run away screaming 'F*CK THIIIISSSSSS!!!!' or other various expletives. Set up correctly, it will handle extended bursts without issues.