Steering speed should always be maxed out. Obviously. Sensitivity though... not really.
If you're using a steering wheel, then naturally you want 100% of sensitivity as you can have full control over your input. But in a case of a keyboard, where you can either go 0 or 1, that at 100% sensitivity will result in effectively not turning of fully turning, means a highly twitchy gameplay with a loss of precision when it comes to steering. Therefore you need less sensitivity, so it takes a bit to reach that 100% input.
For gamepads, if someone is really good with their finger work, 100% will be doable, however usually 80-90% will be better, as it provides just that slightly more smooth input, which can greatly help in controlling the car at high speed when very small inputs are required.