No. "Lift" (in this case in the negative direction, so downforce due to aerodynamic lift) is a function of pressure differential between the air above the diffuser and the air below it as a result of airspeed. Negative lift would be highest when the air below the diffuser is as fast and smooth as possible and the air above it is as stagnant as possible. Therefore, minimal airflow is best above the diffuser (for downforce, not for heat.)

I believe Mazda made some improvements in this area in the S2. Could these panels be added to S1 cars for smoother under body flow?

^ I've done this
I have the S2 oil pan/undertray
it's a step in the right direction but, there is MUCH more room for improvement...throughout the whole underbody

for example:
ferrari 458
http://www.renntechmercedes.com/www/...l/img_0116.jpg
http://www.gtspirit.com/wp-content/u...458-Italia.jpg

this is more along the lines of what I'm doing with a few more pieces

RX8 performance offers this . It isn't full body length it just replaces the plastic oem part . If it were full body length , how would it effect the underside of the car trapping all the exhaust. Has any one put on a full under panel ?.

Is this even necessary on a street car (full undertray).

No tray at all is necessary. The aerodynamic and drag reduction impact of full body undertrays has been proven quite frequently. Most new cars have them now for the mileage benefits.

Ive seen the corvette undertray looks pretty cool. Also wanted to know at what speeds would it be useful?

Technically, 1mph and higher, though it wouldn't be measurable. Track aero discussions vary from car to car, but generally you should be able to start feeling the increased stability from the reduced lift around 60-80mph, depending on how bad your car was to begin with.

Mileage should see measurable benefit around 40+, again, depends on how bad your car was to begin with.

For example, my wife's Mazda2 has NO underbody panels. Between the axles it's just the floor of the car. No splash pan or underbody paneling on either end. Paneling just under the engine bay should be around a 10% mileage increase. Changing your RX-8 undertray won't be a 10% increase, since you do already have a tray under there.

With Miata's they are so unfriendly with aero that a good splitter and underbody paneling you can feel a stability improvement as low as ~50mph. Just some examples. I haven't done specific research on the 8.

Feel free to experiment and start measuring :)

Does anyone know the SCCA rules regarding this kind of aero modification? Allowed in STX?
I read somewhere you can't add anything before center of rear wheels or after center of front wheels, but I don't know the reliability or age of the info.

Are there any heat issues with the temp of the rx8 exhaust, or is that an issue?.

Yes. A 100% underbody paneling is not a good idea. A cutout area under the length of the exhaust will be needed.

I am thinking of adding black corflute to smooth out the under body, should be able to screw it to the rails. The exhaust tunnel can stay unmodified.

I wanted to do this, but my fab guy told me it makes the car unbelievably loud unless you design it with the sound baffle in mind. It basically acts as an amplifier for your exhaust and road noises from what I have been told. This is if you make it out of thin aluminum. They actually make the stuff out of 2 sheets of aluminum with some honeycomb sandwiched between them. I forget the name of the material. It's very light and relatively cheap. It will shred apart if you actually hit something solid though. Anything else would be too heavy or cost prohibitive.