The thing most people don't understand about R/C car aero is this:
The bodies DO in fact, generate downforce. Sure, they don't generate near as much as full scale equivalents, but that's not the point. It's all relative to the weight of the car. For the sake of argument, let's say a 1:1 TC generates 250 lbs of downforce at 100 mph, with a car that weighs around 2500 lbs...so downforce = 10% of it's weight. Now let's look out our 1/10th scale TC's. Average cornering speeds are probably in the neighborhood of 15-20 mph. If a body can produce even 5 oz of downforce, for about a 51 oz car, you have your 10%.
Tire grip is proportional (to a certain upper limit) to the normal force placed on it, either during cornering, acceleration, braking, or a combination of these. I would imagine that R/C TC tires are much more load sensitive than full-size pneumatic race tires because of their relative lack of sidewall or inflation of air. This means that they would require less downforce to generate similar grip increases.