Great read man. I've been through every page over the last few weeks reading this stuff. As a mechanical engineer, i'm impressed that someone is taking the time to actually design some aerodynamics for these cars...or at least seems to be very interested in it...
One dream of mine for years has been around a 'fan protege'...Speaking of rear diffusers and upper and lower air speeds, its surprisingly simple when you're not restricted by racing regulations (if that is the case for you)...
A way to make IMMEDIATE down force is with scavenger fans located under the car...you can theoretically have 14.7 psi of pressure pushing down on the car...while not even moving. Multiply 14.7 by the parallel surface area of the p5...and that is some serious weight. Not thread jacking, but have you ever considered some of this?
It would take a front lip of some sort to get close to the road, and provide an air dam effect that keeps under car air speed low...on its own, that is what you don't want, but by using multiple ducting fans and some tubing with various inlets and two outlets at the rear (used in conjunction with your diffuser design)...you could pull out a lot of air underneath the car, even while moving at high speed...the outlets would also push through your diffuser, creating the same venturi effect that exotic car makers have been attempting for 20 years.
a front of air dam, some side skirt extensions, and a way to seal off the hood...while also using some sort of skid plate under the engine to prevent radiator wash from getting under the car....and look at some industrial supply designs for duct fans...the cfm rates of some of those things is outrageous, and require very little current. A handful of those in the right places with a simple, separate power source...and you're car will hug the road more than nearly anything out there.
There is a reason that nearly every sanctioned form of racing will hand you your hat when you mention anything related to a fan car...its stupid simple, relatively cheap, and ridiculously 'unfair' for racing haha. Its also the magic of 'free' downforce without drag, something that is impossible without cheating. It'll make the car hug the road like a 4,000+lb pig...but change speed and direction like the ~2300lb car you have...even at very low speeds. I'm only partially serious, but has it ever crossed your mind? I know, too, that it goes against the weight savings...but it could be pretty sick nonetheless...
