Mazda Raceway Laguna Seca is a logical choice. VIR as part of CandD's Lightning Lap will offer a comparison, although not simultaneously. Gratton, GingerMan, and Willow Springs have been used variously before. But you have to have participation from TWO competitors to make it a competition, and I don't see a (soon-) past generation Mustang willingly participating.
A next-Gen GT350, possibly due 4/17/14, roughly coinciding with Z/28 initial deliveries?!