I can easily imagine 3 seconds gained on that course by the Trofeos alone. They are a 50 treadwear rating and the the rubber is made in the same single vat as Pirelli's race slicks (I know someone in their Motorsport Division).
So that makes me think that GM is sandbagging, in a good way and rightly so at this point, the true potential of this car. The weight loss could be good for .5 second, deeper braking 1-2 seconds, and the aero for another second.
This new Z/28 should easily run with GT3RS's and base Corvettes all day long.