And realistically, from your plane at JFK to Manhattan, that's got to be a least an hour and a half (immigration and cab/limo). And assuming you'd have to be at JFK just an hour before your flight (and for an international flight that's ballsy), plus with transportation still taking an hour, you just wasted 3.5 hours commuting forth and back to the airport.

That schedule wouldn't work unless you flew to Manhattan. And even then, it'd be tight.