Actually years ago Nascar teams used ultra soft rear suspension to drop the rear spoiler out of the air to get more speed out of the cars. Nascar now assigns the teams the rear springs and shocks they use at superspeedways.
Gt5 is not the 1st game that favors the tail down nose up chassis settings. I've been doing this for a long time. Papyrus Nascar games allowed this method as well. The venerable Nascar 2003 AKA the game that is the basis of I-Racing (66% of the code IIRC is the same) allowed the tail dragging set ups.
^^ This is correct. A lot of you might remember a long while back when F1 drivers could hit a button which lowered the rear suspension and thus decreasing the downforce from the rear wing. This gave them a higher top speed in the straights. Which brings me to the following:
From what I can gather from the OP's data, everything sounds about right except for the downforce part. Taking a look at the top speed data and the downforce settings, it would appear that GT5 takes into consideration the frontal cross section of the car but not the rear wing drag when it comes to top speed. Minimizing the front wing and maximizing the rear wing shouldn't result in such a higher top speed.