Generally speaking, companies list a maximum number of cells for a reason. If you're feeling lucky, then go for it. It probably will work, but it will generate more heat. More heat is detrimental to the lifespan of the ESC. In other words, if you run 8 cells, don't expect the ESC to last for very long.

If you wanna run more cells, I would suggest using a Castle Sidewinder. Not only does it handle more voltage, it also runs brushed and brushless motors.