WAT? The Ring is a super fast pace track, a set up for there will NOT work very well at La Sarthe, Suzuka, and Tskuba. Especially Tskuba actually, thats a super small track and not a huge beast with 100billion mph average speed.
GT4=/Real Life
Actually one of the reasons why the 'ring is favoured by manufacturers for a test bed is down to the wide range of differing corners, cambers and speeds along its length. Yes it does have some very fast 'wide open throttle' parts, it also has a lot of slower sections which have almost constant direction changes. All factors that test the transient characteristics of a car to a huge degree. It also has one thing in abundance that real world roads have, but are often absent from modern tracks (or minimal in comparison to real world roads) and that is camber changes. Throw in some very steep gradients as well (again absent from other tracks) and you have a quite unique and challenging track.
In regard to the comparisons to real world tracks above, the 'ring and Le Sarthe have quite a bit in common, from heavily cambered public road sections to long high speed runs. However your argument could be applied to any two circuits, a set-up for Suzuka would do you no good at Tskuba.
What the 'ring does do is allow cars to that have a good overall chassis and suspension set-up to shine, as without those a strong consistent lap is almost impossible.
Using the 'ring to set fastest laps and then using them to compare two cars is a fun game, but at the end of the day its always going to be difficult to compare them accurately. The same is however true of any track, as unless the cars are run back to back then air temperature, track temperature, wind, humidity and a whole host of other factors will throw out any comparison.
The same is true of 0 - 400m (1/4 mile) times, 0-100-0 times, standing miles/kms, 0-60mph, 0-100mph, 400m/yard radius skidpans, slalom tests, etc. You name it and its will can and will be affected by the above factors, unless the cars are run straight after each other. Yes smaller tracks (and shorter tests) will have lower time discrepancies, but as long as the tracks distance is taken into account it can be factored for. As Joey says, 3 seconds on the 'ring is the same as 3 tenths on another track, as long as distance is factored in they can be compared.
Throwing out 'ring times for the reasons given is pointless, unless you intend throwing out times for every track. The argument that the 'ring does not allow consistent lapping is also a myth, the track gets hired by manufacturers regularly for private testing and the drivers who carry out this testing cover thousands of miles a year at the track (and as a result thousands of laps). Keep in mind that drivers can consistent lap times at the 'ring during the 24hrs (just as they also do at Le Sarthe for the 24hrs), so the argument that lap times are set (and that testing is done under flawed conditions) is simply a myth.
All the above however will never stop people arguing about who is fastest around the 'ring, nor would it be any different if the favoured track of manufacturers was Laguna Seca, Le Mans, Suzuka or Anglesey.
Regards
Scaff