I don't understand the logic behind ignoring an imposed speed. There's a reason you're given a speed. If it's impossible to fly or make a required descent profile say, and some other form of separation will be provided. Otherwise, as an ATCO, I may as well not be sat in front of my radar if I'm going to be ignored.
Had a flight today flown by a major UK airline into the LTMA consistently flying 25-30kts above the imposed speed. I shouldn't need to patrol people's Mode S to ensure speed conformity. The pilot knew full well they were fast, and sounded very sheepish when challenged. The worst example I've seen recently was an Irish airline told to descend at 280kts, and was observed to fly 330kts for a significant chunk of their descent. Frustrating.