f1cdriver, my car was up for tire change today so I was able to look underneath. As best I could tell, the shield-like part you got is but one part of the system; the other part looks like ram air intake opening and duct that bring air towards that shield, I think.
Enclosed is picture of the rear duct/opening (arrows); the front also has a large duct leading air to the shield (labeled x).

 

Good catch, Can...

 

Bob, thanks. I had to go back and forth a few times looking at your picture and mine to finally realize my wheel was off (changing to Pirelli Corsa) and that's why I could see the duct more easily. (I had to look because I am determined to be a bigger car nerd than you! )
I did learn something from this thread. I had no idea the mud guard (grin) is part of the brake cooling system.

Below is the front system, with duct (arrow) and shield (x).

 

Looks like it will still catch air moving front to back without the aero underbody parts?

 

Probably, but I *think* the air catches that way has equal chance of going in all directions (I am thinking of what happens when you are sticking your palm out windows of moving car.)
If I understand your question correctly, when I looked at it, it seems the majority of air movement comes from the duct and the shield merely acts as a guide to direct that air towards the brake. All speculating of course.

That lip that protrudes lower may act to slow down the passing air to prevent (by Bernoulli's principle) a vacuum existing underneath and suck the directed air. Now I am really waayyy out there. :-)

 

Knowing P and the 40 year evolution history of the 911, what you're hypothesising is entirely plausible.

Excellent thread responses both of you!

 

Thank you Cannga for the detail pictures! Much appreciate it.

Here's a picture of the 996 area.

The arrow A, is the square shape a 997tt has, but the 996tt has it as a half a circle, then it leads to B, where it directs flow to the caliper/rotor. At C their is a back opening of the plate, where the duc leads cool air. I'm going to take the car up for a test drive at my local roads later tonight when theirs no traffic and take my temp gun with me. One side has the duc the other side does not. I will see which rotor and caliper is running cooler. If the side with the duc is running cooler in all sectors, it was a success, if it wasent. I can take it off of the mud flap.

 

Can,
You are fighting a losing battle...and nerd doesn't even begin to describe Bob. I swear it's really 6 guys, who all work for P or Google, using the same avatar.

 

f1cdriver, you're welcome. And thanks, I learned something today. Say, aren't you that famous driver everyone is talking about? :-)

ruf_turbo, it does seem that way, doesn't it? :-) Bob does have an amazing collection of data & info.