I think the OP needs to think a bit more about why professional race drivers are so good. Its not because they are using assists or aids. Its because they have practiced many times and have built up an experience and a skill at controlling a race car.
The same applies to GT5 or any racing game - if you want to get better at it, practice. And not only that, but keep practicing without assists. Driving with assists might be easier to keep the car on the road, but it makes it harder to be faster. The assists like TCS and ASM take away control of the car from you in situations where the car is unstable or carrying too much speed or putting down too much power.
Hence, having these assists on means you don't learn about how to control the car in these situations and you learn less about how the car drives and reacts. Not to mention that ASM especially slows the car down, especially in high speed corners.
Some real race series allow TCS and other assists. But the race drivers themselves have nearly always come from a background racing cars with no assists at all. Many professional race drivers have come up from junior open-wheel or touring car series which do not use any aids. This means they learn about car control and driving fast without assists..so that when they are given a car that does have an assist like TCS, they can use it but still drive the car fast knowing exactly how the car is behaving.
Or something like that anyway.
Point is, if you want to become a better driver in the game, its all down to practice regardless of whether you use assists or not. But if you want to vastly improve your driving ability in the game, its better to keep practicing without the assists as you will learn more about how to drive cars on the limit.