No question about it, Viperdays is awesome!
I do have one suggestion for the Michellin Challenge series;
I would recommend reformating the various run groups after the first day. After one full day of driving, it should be obvious if someone in the white, red or blue group is 10, 15 seconds or more behind the faster drivers in that group.
Certain shorter tracks or tracks that have significant no-passing sections present problems for drivers attempting to find space for clean laps. I know that it is advisable to duck into the pit lane to be sent out once again when more space is forthcoming, but it doesn't always work out and give you a full clean lap. Coming into the pits can also break your rhythm and if it takes a lap to get back to where you were before you had the problem with traffic, sometimes you immediately find yourself in the same spot again.
I just think it would be better to try and group people strictly by time rather than by ego, horsepower or anticipated driving ability so that everyone can space themselves out and have the best chance at improving their lap times.
I'd hate to sound like I don't appreciate the efforts that already take place to address this problem - I have driven in enough car club track days where it is incredibly worse. But if you look at the time sheets at the end of the day and see any discrepancies - I would suggest letting that be your guide.