In general terms, I'd say slim to none that you get a GXAR-class car on a Premium reservation; whether they have a BMW or Infiniti at any particular time is anyone's guess. I have had pretty good luck with this station but like most locations the "nicer" cars are very closely guarded. For example, I couldn't get a Mustang convertible on a Premium reservation, even though the Premium priced out more than the convertible during my trip. So, the station's advice to book a Premium is correct, in that that class is the "nicest" you can reserve (unless you can reserve a full size SUV - that may also give you some leverage). If they did have a higher class available, I'd expect you'd have to come out of pocket on top of the Premium reservation.

You may want to check the "Cars at ORD" thread for an idea of what sorts of vehicles people are reporting. I was just there a couple weeks ago and the nicest thing they had was a Hyundai Genesis, which is Premium.

There is nothing "Sporty" in the Premium class at all. Expect large sedans such as the Ford Taurus, Nissan Maxima, Hyundai Genesis, or Chrysler 300. Perhaps you could get a 300C w/the Hemi V8 on a Premium reservation, but not all of the C's currently stocked are even V8-equipped anymore.