Ah, of course, should have known that. Was confused about the line "Indicates if the specified Serial port is ready.". Indeed, while( Serial.flush() ) is the way to go. But I think the name flush is very, very confusing! Judging by the name I would have guessed it flushes the buffer (like it did in old Arduino, but did not know).
So to make it go to sleep fast, use a high baudrate and use while( Serial.flush() ) to check when it's save to go to sleep.