I don't think it would make any difference for, say, an RC car that could fit inside the tunnel. At the speeds where aerodynamics would make a measurable differ

I don't think it would make any difference for, say, an RC car that could fit inside the tunnel. At the speeds where aerodynamics would make a measurable difference to drag, other aspects such as traction and downforce would be considerably more important. I don't see how it would be plausible to measure downforce with this miniature wind tunnel, because there's no strain gauge to be seen. For aircraft of any size this would have more utility. This is because, even at low speeds, aircraft must be balanced (center of mass and center of pressure very close to each other) and ideally won't stall at a low AoA (Angle of Attack). In any case, it would only be practical if you used this with scaled-down models (as used to be the case with real aerodynamics), and it is more expensive and less accurate than doing a computer fluid dynamics simulation. With the simulation, you can do it at full scale in every situation imaginable with high realism and accuracy.