What is the best air/fuel ratio for a C6? I don't know if it matters but the car does have some basic bolt ons such as exhaust, headers and CAI.

Do you have a gauge? That would be the only way that i would even know otherwise your tuner could tell you.

Generally 12.8 is considered the best ratio for maximum power on a naturally aspirated engine but it can and does vary with the vehicle and the modifications.

For forced induction 12.0 is generally considered the "safe" place to be with 10.0 AFR being near the "rich end" for forced induction on gasoline.

Now with E85 or Ethanol the AFR gets quite a bit richer as Stoch on an Ethanol vehicle is 9.0:1 AFR and you want to be about 20% rich of stoch on a forced induction vehicle and about 15% rich of stoch for naturally aspirated.