Yes, that is this you are thinking of, the one in game...
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This was the initial dyno test car, in which many aspects were changed from January to the 1000km of spa. I was actually surprised they did the slant from the intake to the rear wing. When I think of the new Porsche 919 I think of this one, and not the current as it just seems that the front splitter was dropped more for intake to the wheels...
The slight differences you see are things that can be added or taken away. We see similar things like this (forgot the actual name for them) on many DTM cars but taken to the extreme. The FIA allows these to be added to the front to allow more downforce but have to be measured when going under scrutineering to measure the amount of downforce in newtons. Last years R18 e-Tron had two different variants, the one under the same configuration from 2012, and a new test car from the Sebring race called the Long Tail, which the tail or spine of the car was stretched over a few centimeters where the rear diffuser was at maximum length, which provided more downforce.
Here you can see the extension
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And if you look even closer you can see that the air intake was raised to allow more aerodynamic efficiencies and better fluid dynamics with the air.
Just wanted to help out on this post a little, and provide some additional information to those that may be curious.
The original Prototype had some different aero options to the final car for late regulation and rules changes brought on by the ACO. The two biggest of note are the front body work between the nose and fenders, commonly referred to as "legality covers" for hiding the front suspension components, which under the rules are not supposed to be visible. The ACO changed the rules for this area, stating that it had to be one continuous unbroken panel. As both Pugeot and Audi had been exploiting this area stacking the panels at high angles of attack, effectively using them as additional front wings. You can see on the black test car, that there is a "wing" on the leading edge of the upper panels. This is gone on the final competition car.
It is worth noting, that on the current 2014 rules, this are of the car has been "further defined" under the rules, and as a result we see teams starting to exploit this area again for downforce, under "creative rules interpretations". Funny how a less vague rules makes the area that much more exploitable than before.
The rear fin, or "Big Honk'n Fin" (BHF) as it has become known amongst the teams, was developed specifically into the rules by the ACO to help prevent cars from flipping at high speed. The idea that, when a car spins at high speed, it catches air underneath the chassis, and flips it. The BHF is supposed to act like a gurney to slow the air over the top of the car, and keep it planted to the road. It was feared that BHF trailing down toward the rear wing element might cause a vortex over the top of the fin, and cause the car to react abnormally in a high speed slide. So the rule was changed requiring the BHF be a consistent level line along the reference plane of the cars dimensions. The actual effectiveness of this solution to prevent high speed roll overs is still heavily under debate, as plenty of cars (including the Pugeot) have flipped in a high speed spin, but...it is there for better or worse.
Under the rules, each car must have two sets of body work. While they are commonly referred to as "high speed" and "high downforce" options, it effectively works out as "normal circuit" (high downforce for the WSC Series Sprint Races) and "LeMans" (low downforce for improved straight line speed) aero packages.
The front elements on the car are called "dive planes". Also often referred to as "strakes" or "flow conditioners". Why these are not modeled on the "test car" in Gran Turismo, I do not know. Clearly they were on the car at its launch.
The air inlet was also changed from a fully open inlet hoop, to a portal inlet, likely for flow improvements, improving the way the inlet catches the air off of the front wind screen.
One other area, though it doesn't appear to be modeled, just textured, is the front wheel covers around the lights. You can see a small black hole on the outside of the fender, towards the top of the light, and on the inside of the fender, down low near the flat floor. These are the intake (inside fender), and outlet (outside fender) for the power system on the LED headlights.
Also on the inside, is a black area toward the front of the inside fender. Though it isn't modeled, this area actually had additional brake inlet cooling, as the change to the nose panels, facilitated a change in the cooling inlet location to maintain optimal cooling performance.
I really wish they would put more launch cars, and final competition version cars into the game. The evolution of the development is a fascinating one to me, and I appreciate the form and function of the designs evolution.