Downforce is common practise in racing. The reason why racers go for downforce is that the force required to slip the wheels is increased, which results in better braking, and probably steering as well. 

But when we talk about a 1:24 car traveling in approx 15km/h? Is there any downforce to speak of? I'd like to find out.

I need more force on the steering wheels, they slip a lot in the turns.  

I have, however, experimented with weight. It gives the same effect as downforce except for that the car has to make of for inertia. As is common with a four wheel drive, turning in high speed "forces" the car to steer less. There ought to be a sweet spot there the slippage due to low friction is minimal, and the slippage due to inertia is minimal. 

To be updated.