This is not completely true - to my knowledge. Increasing the ASM Oversteer setting has no direct effect on the cars performance while understeering, only a secondary effect. Nor does increasing the understeer setting increase the oversteer experienced by the car.
These settings work to correct a cars handling. Increasing a setting increases how strongly the driver aids will interupt your input to straighten the car out. This is all.
for instance - increase the understeer effect means the electronics will more violently intervene (with respect to a lesser setting) to try and remove slip from the front wheels.
Personally, I turn both ASM and TCS to zero. I'd sooner have full control and have to deal with understeer/oversteer on my own, than having the computer do my work for me. There are instances where you may desire some oversteer to correct your exit from a turn (in drift or grip scenarios). Having driver aids activated may inhibit this, or make it more difficult than it need be.
In short - I go faster with them off, and I definitely drift better with them off.
If you learn decent car control skills you wont need them.
