Did you read my post?
When you finish reading it, I must add that I set the DFGT to Simulation, with 6 FF and NO assist.
And, if I may, do you have GT6? With wich setup did you drive on it?
I read OP and some others not yours.
Now...I of course have Gran Turismo 6 Anniversary Edition (GT5 Signature Edition...GT4...etc...also played the first Gran Turismo back in 1998) just so you don´t have a doubt.
After reading your post...I have a Logitech G27 and I have no problems trying to dominate the Pagani Huayra with the right amount of gas at the right moment.
My wheel is set to simulation and FFB to 5 (since more than that gets to heavy and feels no natural at all). Then I always drive with all assists off and ABS 1 because at 0 the tyres lock no matter if you press the pedal a little..without ABS it just works as a button...ABS 1 gives you some progression while braking and feels better.
To understand some behaviour of the cars in GT6 you have to understand how physics works in it, for example in this game the weight distribution is way important and it can make the difference between controlling or not each car. (while braking and turning mostly)
Also the power of the cars, where the engine is and from where the traction comes. (front/rear/all).
Having in mind that, in GT6 the cars behave way too "mechanically". You can feel where the weight is all the time, where the power comes and because of that you can react in time to make adjustments and such things if something goes wrong. But also you need to be as technical as possible while driving, with more control of your feet and hands...some cars are easy to control and some needs more practice but at the end you can dominate those "hard" to drive cars (and even if they are not as realistic as you believe).
Some cars have a learning curve...this is not a game that you grab and then you press the gas and just turn in the curves...it will be too easy and too boring if it is that way.
I have the Huayra, is not easy to drive but once you get used to how it behaves you can control and dominate it. It takes patience and practice.
Also if you think the Huayra is hard...try to take curves with the Lancia Stratos even at low speeds. At first that car was a pain because it reacted in a so "weird" way...but since the engine is almost in the middle of the car, the car itself is small, and it has a lot of power...as I said...in GT6 the cars behave in a more "mechanical way", so it will respond as the physics are made to it.
It can be less real o not...but is one thing that you need to get used to...after a while you can handle it and the same will happen with the rest.
A lot of cars needs refinement in the physics though...too many cars...too many variables. It is hard to make everything 100% accurate/realistic.