Actually looking at F9's route map, it appears that their longest existing route is 2400 miles (DEN->ANC) according to WebFlier. That will shortly be eclipsed by their new Costa Rica route (2410, DEN->SJO). With BUR->JFK on B6 a stretch at 2460 MI (though takeoff weight limits for BUR might be part of the problem), it's probably a decent bet that F9's 320's won't be serving ANC or SJO.