Not quite dynamite, you'd need 2000' sep at FL440. The reason those higher levels don't get much use is because aircraft don't ask for them, those up at say, FL450 for example get direct routes everywhere - due to lack of other a/c at that level. Sod's law does dictate though that you will have a conflict somewhere, it happened to me at FL450. FL660 I'm glad to say was "clean" of other traffic on the only instance I've controlled an a/c at that level.