On the LAX-MIA and SFO-MIA routes, the best would be to get a 763: the 'F' section on these planes has a long-haul J-type configuration, which means that you get a very comfortable seat but still get TPs for F. This beats the heck out of a CE trip by a country mile.

Incidentally, was it really a K-UP fare or was it a discount business class fare (Ixxxx fare basis)? IME, the cheaper fares to CUN (and to central America, for that matter), are I-type class fares (even though you fly F on domestic sectors). If the 777 was offered on LAX-MIA (albeit in business), the odds are that it was an I-type fare, as K-UP fares typically do not allow travel on the 777 at all.