100 % of the vehicle capability:
This would be driving at a speed that allows for no error and no unknown irregularity in the course or conditions. The vehicle will more than likely be sliding sideways. What you were looking at through your windshield will now appear through one of the front door windows. If sanity is intact, apprehension is present. Requires a great deal of skill and some luck; this is reserved for closed-course race car driving, such as qualifying for competition with the pole position in mind.
If you are conducting a training program, you should be pushing drivers to a minimum of 80 %.( my opinion). Also, it is my opinion that if a driver can't use 40% to 45% of the vehicle, they should use walking as a mode of transportation.
Although it is not recommended that you use 100% of the vehicle on purpose, there will come a scenario, maybe not driving through a corner, where you will be required to use all the car has to offer, in a very short period of time and space.
All The Above Can Be Scientifically Measured
There is now equipment available that can be plugged into the vehicle's cigarette lighter and will measure the amount of force the driver is applying to the vehicle. Depending on the level of sophistication of the device, it can be inexpensive ($190) or somewhat expensive ($1200). The device will measure the force (Gs) exerted on the vehicle and how long that force is applied to the vehicle, which in turn tells the instructor how much of the vehicle the driver is using. The high end devices will transmit the results to your computer outside the vehicle. All the results are downloadable.