I think that you have to distinguish here between "works" and "supported". I have also used an NM-8A/S above its supported speed. It might work, but the day it stops working you cannot complain to Cisco.
I guess that if you are running it as DCE, it will not allow you to overclock. But as DTE, they cannot control the clock. If it works, fine. If it doesn't, no support.
Kevin Dorrell
Luxembourg