At the end of the day, as mentioned before, speed control is a form of separation and if you deviate from that speed it can be at the worst, dangerous. As controllers, our end of the bargain is to allow to trade off in the speed/height equation and not ask the impossible. It is also the responsibility of the controller to follow it through, ie, if a plane comes on doing 325 indicated from an area sector to the TMA, unless we change it you could, and should hit the hold at that speed, so we must reduce your speed if necessary to an appropriate holding speed or whatever we can co-ordinate with the approach controller. The bottom line is please fly the assigned speed and ask before doing anything else, it is after all, your arse you will be saving........