NASA limits their events to 15 cars per mile of track. You do the math, if the event fills up it will be the same as 90/94 in rush hour. MFBA limits their event to 120 cars total or ~30 cars per run group. Which is much better.
NASA does offer open passing group, not sure if it's a good or bad thing. MFBA does not, their fastest group is low 2:30's to low 2:40's.
I ran with MFBA group once before with S2000, it was pretty good a couple of road blockers but not too bad. In any case my new car has enough hp for the straights so road blockers shouldn't be a problem.