[wildman10]

Guys, thanks. I'm rushing around for a while so some quick points until I can think over and research your info:
* I've changed my format and wording on the 4 ducting methods to be clearer/less confusing (table in Word gone bad here).
* I, too, am wary of the effects of Re for cars rather than aircraft, where our viscous forces are relatively more important, and I know there are significant differences for heat transfer. However, I have seen nothing on the lines of "x works in aircraft, but on cars with lower Re that doesn't work and we have to do y". Our far lower values of 1/2RhoV^2 make some things just not worth doing, but that is a different issue.
* As we are going into greater depths on RX8s, I'll add some more detail. The 13B REW conversion means that I have a clean sheet for the engine bay; I've put the battery in the boot, moved the engine bay electronics to behind the bumper to make way for the turbo's air inlet components and thrown out the RX8's oil and coolant radiators and piping (the coolant system is now inadequate and I have to fit new oil hoses where a kit with 2 radiators costs little more than the hoses alone). I bought her as a done project that turned out to require drastic surgery (the builder is an engine rebuild trader; he may know about the engines but he is useless at conversion design and bodged all the work he did). Her terrible mpg (the ECO gadget on my standard RX8 shows less than 20/16 UK/US mpg overall and as low as 7.8/6.6 UK/US mpg on track days) perversely makes eco measures even more important.
My suspension will lower her by 18mm/0.75in and my front splitter and sump guard will add 10-20mm/0.4-0.8in. I don't want to go any lower/stiffer as UK roads are in poor condition.
I'm looking at smoothing the underside but have to give priority to cooling the gearbox and am wary of trapping heat from the exhaust box under the boot/trunk (I've noticed the boot can get warm on standard RX8s).
I'm looking to fit a rear wing and a splitter extension (takes 1" splitter out to 4") for track days, leaving them off for most normal driving to avoid damage and unnecessary drag. I may fit a diffuser but am sceptical of any benefit.