The relevance is that we would know how fast the car truly is, given all that the manufacturer has put into the vehicle. I don't think it is by any stretch of the imagination pure coincidence that Tommi Makinen throws the STi into corners with the abandon for which the top rally drivers are famous. This kind of aggression is never seen by HvS (though it is graphically on display by Suzuki in the GT-R, as well as Iida to a lesser extent after the first portion of the track). Clearly, Subaru has engineered the STi to be quick and responsive in that manner, whereas Jim Mero using the same tactic might be picking up bits of Corvette debris in the nearby woods.
Is the point of such testing to find a method of consistency (in a place which by its very nature can be inconsistent from one lap to the next) of comparison between cars? Or are we looking for how fast a car can tame the demanding Nordschleife? Put it another way, why is ~9/10ths lapping by a driver who is for sure much more familiar with Porsches than your average enthusiast anymore relevant than factory tests where the driver is for sure familiar with the car in question? Using a control driver with decades of experience may be a good thing for consistency, if it is your desire to see almost consistently slower times than factory pilots.
You do raise a legit point about relevance: How is HvS's decades of experience, with a closed Nordschleife, relevant to the average enthusiast driver who has no such experience, and no such opportunity to drive a closed Nordschleife? How many private individuals have closed down the 'Ring for their own pleasure? Last time I heard it was one; and he didn't even do his own driving (but hired a VLN ace to do it).
I wouldn't necessarily say faster makes the car better than another, if that's what you mean. On the road, where 99% of buyers will spend 99% of their time, there are other things that make a car "better" than another IMO.
