It does, but does not explain the downforce vs. speed relationship that I seem to be observing. I have enough understanding of fluid dynamics to be past the very basic description you've provided, but right now it's quantifying theories and data that's going to be fun/hard.
My understanding of intermediate-scale fluid mechanics (such as 1/28 cars) is that it's right at the boundary between viscous flow (bugs, germs, etc.) and inertial flow (real cars, planes, etc.) and there's a combination of both effects that's taking place. I went and talked to a few professors about it, if you do the math then the speed, size and shape of a Mini-Z lands it squarely in the "intermediate" Reynolds-number territory. This should mean that the boundary layer around the object is quite sizable compared to the object itself, and air hitting the front of the car doesn't actually hit the car but rather hits the boundary layer some distance ahead of it (1-3cm, depending on speed?). That makes each body a lot "rounder" than it looks, but also significantly slows down any air going under the front bumper.
The Reynolds number thing might be why the front diffuser in particular works. In the local area of the front bumper the Reynolds number gets smaller, since the diffuser is smaller than the whole car and the air the diffuser is fed is significantly slowed down vs. the vehicle's actual speed. So given that 1/28 cars start out at the "intermediate" stage, one could expect that the lower-Reynolds front bumper area (smaller size still, lower air speed as well) would predominantly follow low-Reynolds number flow characteristics, where drag/force are proportional to speed^1 (linear relationship).
Meanwhile, a raised rear spoiler is outside the boundary layer and is subject to air moving at pretty much the speed of the car, so the intermediate-Reynolds flow holds and you get a not-quite-linear relationship between drag/force and speed. That's the theory that I've come up with.
Meanwhile I made another front diffuser, this time for a more "standard" racing body, the Ferrari F430 GT.
The angle and volume are more aggressive than the 911's diffuser so I'm hoping to get more downforce on the front end, period. I do think that I may have hit the limit for diffuser angle though, the angle in these pictures seems to be too much, and I got some weird handling inconsistencies when testing the car. I took off the body, straightened the diffuser a bit (reducing its angle), put it back on the car, and immediately everything got consistent again. This is something else I want to explore, the fluid flows here are just so interesting and hard to figure out I think I'm gonna need software on top of the wind tunnel, just like you've suggested Kaze.
Meanwhile I built up the rear diffuser and central underfloor too, but these sadly don't seem to make any useful downforce other than compensating for their own weight.
I'm guessing that my ride height might be too low to allow airflow under the car (boundary layer stuff again?) because the pieces don't make any appreciable downforce other than to compensate for the weight they add. Rune's rear diffuser actually adds grip, he says, mine pulls its own weight but doesn't help out on top of that.
Hobby turned academic project is a great excuse for spending time on the hobby, lol. Glad to be (somewhat) back, happy to share any discoveries with you guys since this "initial-period" research isn't going to spread by word of mouth alone.
