Are you sure these weren't C5 Z06s? The reason I ask is because the C5 Z06s have badge on the side that say 405hp. The C6 Z06s have a badge that says 505hp.

I've seen stock C5 Z06s run anywhere from consistent low 13s@105 to mid 12s@110. A few weeks ago, I saw a stock silver C6 Z06 rip a 11.7@125mph on drag radials. I was in absolute amazement and apparantly that's rather slow for a stock C6 Z06. If I had $70K to spend on a car, that would be it.