I've tried researching this on the road, and came to the conclusion that on a really slippery, greasy roundabout I can just about detect a drop off in wheel torque at extreme understeer, but it's not something you'd probably notice without looking for it, and it appears to only occur (on road tyres) at very high slip angles.
I also experienced it ONCE in a race environment - actually at a sprint meeting in 2006 at Lydden. It was a damp, greasy day, and the track is partly used as a rallycross sort of thing, so the tarmac is generally filthy. On ONE of my laps (I did about 8 that day, which sounds rubbish now because I do more than that in one session) at the hairpin when I got understeer [due to low tyre pressures, inexperience, trying too hard and the cold/greasy track, whilst on rain tyres] the steering went really light for a bit, but the natural reaction is to sort it out and reduce steering input until the understeer goes away.
It has never occured whilst on slick tyres, even when understeering in the 'dirty air' following another car, but that might be because I haven't put myself in such a ridiculous position for it to happen, simply by not being brain dead.
In short - I don't think it's worth bothering with, and nKP overdoes it. Not sure about rFactor because it's terrible anyway. It only applies to treaded tyres as far as I can tell. And I can tell if I'm understeering in LFS anyway.