"Easy fix. Put in the C16, turn up the boost. That turbo is a dog until you get to mid 20s. There's most likely nothing wrong with the car. "
I was about to say the same thing. A T66 is not going to make much more power that a T61 on pump gas on our 3.0 motors. A turbo like that doesn't really start coming alive until 25psi, especially with stock cams and head. In other words IMO, it's really needless to have a turbo of that size and only run pump gas. However, if you decide to run some high octane, I bet you will get a pleasant surprise, especially with the standalone tuning.