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You have to be very careful when transferring your knowledge of airplanes to cars.
Airplanes (especially sailplanes) are highly optimized to begin with--they've already taken care of a lot of the base drag by tapering (boat-tailing) the wings and fuselage. That hasn't been done with cars (to the same extent). Accordingly, it is worthwhile *on an airplane* to spend a lot of time on the nose because they've already taken care of the all the parts that are worse.
Second, when it comes to wings (and even the nose), the airplane folks care about lift and are usually trying to either maximize lift (which we're trying to minimize if we even care at all) or maximize the lift/drag ratio. We're (usually) just trying to minimize drag. The difference in goals affects the approach one takes--it may well be the case that the front of the wing (given what I've said above about the trailing half of the wing) is (now) the most important area from the point of view of *lift*. It is clearly *not* the most important part of a typical car from the point of view of *drag*--the rear is because (generally) the largest single drag compoent is base drag.
Third, when you consider the front of the car, you can't just optimize that part in isolation from the others--as you point out with airplanes, what happens there affects what happens downstream. Aerodynamicists treat the front of the car as an integral part of the whole and when they consider mods to the front, they consider what it does to the air flow (and therefore the drag and lift) elsewhere. It is often the case that by making the front have a lower Cd (considered in isolation), they affect the airflow around the car (and especially under the car) in ways that *increases* the drag elsewhere. You have to treat the whole car as a *system* and optimize for overall reduction of Cd.
Fourth, for the reason I just mentioned, there is likely no one *optimal* front end design--there are front ends that work well with particular "back ends" (the part from the front axle all the way to the rear) and front ends which don't. The front end design you pick will depend, in part, on the back end you already have (or plan to make). This is true also with airplane wings and airplane fuselages: you can't just take the front end of your sailplane and put it on a Cessna and expect it to produce good results. Ditto for the leading edges of wings--the whole wing, the whole plane has to be considered.
--Steve
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