Hmm...most a/c won't spool up the engines until ramp control clears them for pushback.

don't need to pushack when at an FBO

Or just:Pilot: "Toronto Ground, Air Canada 123 requesting IFR clearance to Miami"YYZ_V_GND: "Air Canada 123, cleared as filed, contact when ready to taxi"Which is what I'm usually getting :-)

And at a busy airport they still have to clear 60+ aircraft per hour which requests are of course not nicely spread out but clumped together.

You still had better wait starting engines until you have clearance. It COULD be denied or you might get told you have to wait a bit...Last month in Curacao I heard some pilots asking for clearance for their outbound leg while still taxiing to the terminal after landing. Talk about being in a hurry to get away again :-rotor

Hi MarkI thought it might be useful to add some other insights for you.Unless your f/p is a no-brainer the controller will be reviewing itfor several things such as have you entered the correct ICAO codesfor departing and arriving airports, is your en route altitude proposed correct for the direction of flight (not always obviouson north/south routes - eg: miami is slightly west of toronto 80longvs 79 degress).Are there local preferred routings to add prior to your first waypointand so on.This is normally what he will be reviewing before deciding to ask you to ammend your f/p or clearing it as submitted. If he is not the GND or DEL controller he will be performing other ATC functions beforeadvising "clearance available advise ready to copy".As someone else mentioned earlier GND/DEL are often the positionsstudent controllers start up at and sometimes they need a bit of time to assess these issues.Hope this helps.