In my training, our instructors had a fight in front of the class on exactly this issue...
Student: "MATS isn't clear on the subject, can we abbreviate Foxtrot Foxtrot Foxtrot to triple-Foxtrot?"
Instructor 1: "Yes, after first contact with full callsign that would be acceptable"
Instructor 2: "NOT IN MY SIM!"
Common opinion was Instructor 2 was a little bit of a jerk (Kelpie concur?). His control technique was to aim all aircraft at the same point (14 NM final) and use speed control to sort out the sequence!
Personal opinion? If you want a slick callsign, use one with single-syllable letters. Golf Golf Golf -- now say that one fast three times over.