Hmm...
Alright, let's correct some thinking here.
Cars are wedges. The shape of a car provides downforce. Thus, the faster a car is moving the more downforce acts on the front of the vehicle. At the same time, the rear window and trunk of a vehicle often create a vacuum behind the car as it moves through air, and this vacuum actually produces lift in the rear of the vehicle aswell as drag. Spoilers and wings not only act to produce downforce, but reduce lift and 'spoil' the vacuum drag. This will cause the vehicle to 'tuck down' as you hit higher speeds, and many enthusiasts will know the feeling I'm talking about.
In addition, the faster a vehicle travels the more inertia it has, and the less its overall weight is involved with its velocity, wind resistance becomes much more of an issue - this is of course once acceleration has started to plateau. The added downforce won't make the vehicle slower at all.
The whole "wing/downforce/RWD" arguement doesn't make much sense - mainly because traction is most often lost off the line - where you will have no downforce. At speed it is extremely EXTREMELY rare to lose traction save for uneven surfaces. I say traction due to torque because that is the only situation that drive wheels are involved, and thus RWD is an issue. Losing traction around corners is more dependant upon weight distribution. I find it important to note that RWD cars carry driveshafts that run the length of the vehicle, differential assemblies and halfshafts, often now CV joints connecting the engine to the rear wheels - this additional weight in the rear of the vehicle will tend to negatively affect performance around corners... however this doesn't mean that somehow RWD cars get more benefit out of a spoiler/wing.
Lastly, strapping a basset hound wouldn't add 60lbs of force to the vehicle, it would add 60lbs of weight, which would have a negative effect the cornering of a vehicle. It would also negatively affect your acceleration, where downforce only comes into play at speeds where it is preferred.
