As has been said before when others have asked a similar question, if you want to be 100% sure of getting a Prestige Collection car, then you need to book one.

Maximum Presidents Circle "guaranteed" upgrade will get you now, it would seem, to a Full Size, and previously to a Maxima/Taurus. If you got anything better than that, them it was your lucky day, based on availability vs reservations. People often were lucky, but there are absolutely no guarantees, and if you arrive when the lot is full and there is no shortage of cars, you won't get upgraded at all.

Sorry if this is not what you wanted to hear, but it's how it is.