I did it gradually, in GT4 I never even realised that the aids were on, when I discovered them and tried the cars with them turned off I found with practice I had much more control over the car and could drive faster.
It is probably worth turning them down gradually, but it depends on the car. Many cars are really easy to drive without the aids, others are very difficult.
Lamborghini Murcielago LP640 - This is a good training car of a very oversteering (tank slapper) type car, you will need to use throttle under braking and taking your foot off the throttle needs to be done extremely smoothly, countersteering will be your best friend when you lift the throttle, and for 90% of the time you will always have atleast a small ammount of throttle applied to keep it stable.
Driving that car without aids is really difficult, if you can learn to drive that then every other car in the game will be easy. The strange thing about the lambo is that if you can learn to drive it, you will run very fast laptimes because of how easy it is to avoid understeer.
Personally I hate the car, it is insane. ha.
Skid Force Recovery - This gives you more grip and will make you faster, turning it off though in my opinion will improve your racing enjoyment after you adjust.
ABS - This does make you faster, turning ABS off is not the most realistic simulation of how non ABS brakes should act but some people like it, you will need to change the brake balance settings away from the 5/5 default. Personally I recommend leaving this on.
Traction Control - In most cars it slows you down, turn it off.
Stability (steering help) - This can make you faster by essencially doing the precise steering adjustements for you, similar to what Shift 2 seems to have built in. It can help some cars but if you can drive the car decent without it then turn it off.
Active stability ( ABS based braking stability ) - The system that applies the brakes to help keep you stable, this thing will slow you down a LOT, this is the biggest speed killer in the game, never use this on any car.