I recently purchased a Diablo GT2 for 300,000 cr. I fully tuned it up to 1006 HP, but it only goes around 230mph and can barely steer. Anyone got a good tune?
As a rule of thumb you should get used to a car stock before adding power, this gives you and idea of the balance the car has (you might have already done this ). As you increase power, in your case to the tune of 400 or so HP, you can see where the car needs improvement with the added umph.
Generally I like MR cars to be unstable during corners. Usually i make the front softer than the rear, this requires you to steer mid corner, and on corner exit with the throttle.
To compensate for stability issues, adjust lsd power on corner entry to reduce the car being unstable during braking.
I dont have the game, but this is how I'd set up my NSXs in GT5. Never been a fan of over powered supercars, they're already ludicrously fast and powerful out of the box. Hopefully this helps.