Major Cong, did this occur outside of controlled airspace? I'm sure you are aware that 250 below 10 is mandatory outside of CAS. Although there is always the temptation to wind it up when someone is behind us. Some ATC units will waive the speed restriction when they do not have the right to.
In my company, the descent profile is normally designed with 250/100 but if circumstances require it, there is nothing to stop us exceeding it legally (although the little birdies hitting the windscreen at 314kts are reckoned to a bit more dangerous than their slower friends).