All a CF is a guessimate of what the car should do at sea level. The number you got on your Z was based on a guesstimate of at least 20% over your actual dyno. You actual dyno is about 302rwhp. That is a significant guesstimate which I don't trust . Anything over 10% on CF is not trustworthy. Your big HP numbes makes you feel good. But that number was attained with 1.20 CF. So 1/5 is a guesstimate.