So far, I've been using just a few tweaks to the default settings. I didn't think they were helping all that much until I tried them on my GNX and knocked 2 seconds off my lap time, bringing it down to around 2:09.240 on Road America. Here's what I'm doing right now:
*note- use only the forza rear wing even if other upgrades are available
Tires 29/28
Camber -1.0/-0.8
Toe 0/-0.2
Springs take 20-40 lbs or more off the back, honestly I'm just eyeballing it!

AERO -/max
Like I said, it's a very few tweaks, but it definitely helps me move these types of F/R 1000hp monsters around.
The GNX is posting the worst grip numbers but doesn't show it on the track. It's holding down the number 2 spot ATM The Challenger has the power (1097hp) and the grip (both g benchmarks over 1.01), yet still manages to set poor lap times, same thing with the Cuda. The fastest thing on four wheels has been the 850 horse 2000GT, easily capable of hitting 211mph on the straight. I can't figure out why the brakes suck on it so bad, otherwise it would blow the doors off the T/A. It may have something to do with the skinny tires, but it still seems to me that it could do a little better. They're really bad!