As you increase slip angle, the torque required to turn the steering wheel (or keep it turned against the self centering torque) increases until it peaks, at which point you are using the tyres to their maximum - any further increase in steering angle would increase slip angle beyond optimum, reducing cornering force, and also reducing the torque on the wheel. So yes, it DOES get lighter (but not REALLY light), when you induce understeer.
Then, if you're a really bad driver, and you keep turning the wheel hopelessly believing that more lock will somehow cure the understeer, the wheels no longer rorate 'freely', but have to hop across the road surface. Anyone who thinks this is normal needs driving lessons.
LFS simulates all of this afaik (apart from the hopping, but race drivers don't need that simulated because, of course, we never do that. Do we?). It's subtle, and can only be felt if you have Windows Game Controllers set properly (see other threads), but I feel the peak of torque required and the decrease afterwards. Maybe someone should measure it with a Newtonmeter or something :S