Effvee, great craftmanship. Im not trying to put you of, i like what u r trying to achieve. Im just giving you a different angle, playing a bit of devil's advocate...
considering the decklid, i have some hesitations... I think the effect will be comparable to just normally opening the decklid a bit (not stand offs), which is a real no, cos the hot air leaving the engine is getting trapped by the high speed air passing over the decklid thats gets turbulent, and because of the low pressure created by opening the decklid it gets drawn into the engine compartment. i wrote the above only after looking at the pics, and using basic thoughts. the other thought is that the air will just run along the inside of the decklid and escape through the lower louvres. If this happens it will be possible to "steal" some of the air going into the fan (stock) because of reversed louvre amount and positioning.
s for the metal lip, i belive it is a great idea, but i suspect that it will cause too much drag, and stress on your mechanicals to reach and sustain high speeds.
rule of thumb for cars aerodynamics/mechanicals relation.
A. Up to 80 km/h the engine's work is spent to overcome rolling resistance and friction. any speeds above that, the engine is working against the aerodynamic resistance caused by the vehicle's shape.
B. Aerodynamics start working at around 80 km/h depending on vehicle's shape and size (justifies point A) but only air flowing above the vehicle. A speeds over 120 km/h downforce starts to be generated. so the underside will play some role.
Effvee, according to the dirty air concept, this would happen if a beetle was lifted roughly at half of the average of front and rear track. but because every one going on the road is lower than that, it is more important to aid the venturi effect created between the road surface and the car's belly.
Also, very important air is not flowing around, above the car. the car pushes through the air, similarly the air passing under the car is considered static that then get disrupted by the vehicle, pressing it against the road (front splitter) and theen allowing the air to smouthly transist back to a more stable state through a flat bottm, preferably along with a diffuser at the back.
Sorry for the long post. god this gave me a headache...
Chris