Dr Phil.....

On the engine power.... Im sure it has more than 500, but not because they prepped a special one, but because GM has
always understated their V8 performance engines output. They
are very conservative on claimed HP, not just for testing sake, but in the cars they sell.

As a pure guess deduced from owner testing dyno results of
other GM performance engines, the Z06 may have something like 470-485hp AT THE REAR WHEELS and close to 500 ft lbs of torque.

As for who did what and when and how fast. I am willing to take Magnussen at his word. He has the credentials to back up what he claims, and so far no one has shown him to be
suspect about anything.