As I'd mentioned, I'd probably not really go for more than +/- 1-2 toe.
The "Nanny" settings I'm talking about are in your settings window under "Driver Aids". There's ASM (Active Stability Management) Oversteer and Understeer settings which try to correct your cornering by cutting power in and out to stop you sliding, but slowing you down considerably in the process. The default settings are 10 and 10 for these, but you seem to have raised them to 15 and 15 meaning you can't turn the car at all without the electronic babysitter "helping" you out.
The TCS (Traction Control System) cuts in if it detects you're breaking traction on your drive wheels, in this case your rear tyres, which, in a 960HP car without good throttle control in the first place, is pretty easy to do!
Sadly in GT4, every time you buy or win a new car, these aids are automatically on and set to 10,10,5 (or 7 in some cases) meaning you have to go and turn them off to 0,0,0 (maybe 1 on TCS in some cases) on each car you own in order to be able to drive properly without the electronic "Nanny" kicking in and rescuing you.
You'll be able to tell how much time you're being robbed of by looking at your driving display, there's a little 'Blue Triangle' icon that'll flash on and off if the aids are being called into use. If the blue triangle is there more often than not, you're potentially losing a lot of time.
Overpowering the car by increasing it's power output by almost half-as-much-again than it had already it just going to make the aids more likely to kick in to try and control the car on your behalf and thus your extra power is essentially wasted anyway.
I posted this for another user's benefit recently, but it applies here to and is seriously worth bearing in mind here, as you'll struggle later on, especially when you hit Professional Hall GT World Championship where you can't simply overpower your opponents, if you can't beat a somewhat simpler event like All American Championship in a massively overpowered car:
...However, it does somewhat prove the old adage that "power is nothing without control" an thus a bit of "back to basics" might help.
Did you spend much time on licence tests in GT4?
They're worth revisiting if you settled for Bronzes all-round first time through. They'll teach you the finer points of how to scrub off 1/10ths of a second for individual bends and hone your skills in doing this in slower cars. Fishing for 1/10ths in slower cars is harder than in faster ones, however, once you can do it in the slow cars, the technique you've gained will help enormously when you come to to faster cars. Additional power will only get you to the next bend quicker. If you lack the technique to get round a bend quickly, you'll lose any time that you've just gained.
Check out the Missions & Licence Tests subforum, there's some good guides there to improving. See if you can get all Silvers or Golds in some or all of the tests. Sure it can be frustrating to chase down that final fraction of a second, but all the time you're learning about car positioning, braking distances, weight shifting, throttle control, and ultimately it'll make you quicker & more consistent...
Some people view the licences as an unnecessary hindrance to the ultimate goal of going racing, but they do, in some cases not obviously, teach you a lot about the car dynamics (and yes, we know they're flawed in all sorts of ways in GT4) and control.
In the same way that you or I can't go out and fly a Jumbo Jet or Stealth Fighter without initially learning some fundamentals of avionics and getting some stick-time in a substancially less huge and/or powerful little plane first, so too there's a reason that people aren't taught to drive in NASCARs, IndyCars or Formula 1s without having some experience of driving and car control in something considerably more basic first. 👍