Expedite is all very well, but can mean different things to different people. The Tristar example has already been given, but an A340 in the climb en route HK also springs to mind. So "Expedite" in itself does not ensure separation. I prefer to find out what rate of climb the a/c is able to give me through the relevant level and then specify that or not less than whatever I need. Then at least you are actually CONTROLLING the situation rather than relying upon some pilot (or computer) interpretation of "Expedite".