I'm another Watkins Glen participant who used up the last half of my RS rear pads from only about 10 hours of track time. When the rear pads were gone, braking effectiveness dropped dramatically. It felt like at least a 25% loss of braking ability.
I remember seeing SteveS' rear pads having the same results at VIR. The worst part is, I have yet to see squealers (a.k.a. sensors) on the OEM rear pads, so you can't rely on a warning squeal - it's the real thing by the time you do hear it.
Edwin brings up a good point about the parking brake issue too.