With the options you gave, this seems like is a no-brainer to me. A C5 Z06 is WAY faster on a road course than anything else listedexcept for the C6 Z51, which is about the same speed for more $$$. Let's look at where these cars are classified in SCCA Club Racing:
C5 Z06- Won the SCCA Runoffs the last 3 years in T1
C6 Z51- Currently runs T1. Competitive, but slightly slower at the moment for lack of development.
Lotus Elise: Runs T1, but is uncompetitive.
Every other car mentioned runs in T2, a slower class.
Weakest point of the C5 Z06: Front hubs. For a track car, you MUST have spares, and unfortunately they're $600 each
