From Stihl an 8 is all you are going to get. There might be an orgeon replacment, in 9.. maybe. For milling bigger wood you are going to be much happier with a 7 or 8 in any case. You need torque more than chain speed.
Be careful milling... you don't want your saw "on the edge" - "the edge" is where the piston melts and you get really mad.. Be very conservative with carb settings and fuel mix. IMO.. It needs to be DETUNED to about 80% (just a guestimate) power, not maxed out. Milling is a full continous power operation often for an entire tank - not what the saw was designed for.
A sharp chain is far more productive than incrementally more HP... My second pass though a hard 12 foot x 24 inch log is nearly half the speed of the first and by the third, I need to sharpen the chain as it's dull. Close to the edge + "dull" chain = dead or stressed saw.