As 9C1fanatic stated, there is a significant amount of Torque Management placed on our cars. One such layer of managment is the "Braked Torque Limit" table which limits the amount of torque the engine is allowed to make for a given Throttle Position and Manifold Pressure (engine load). So when the car sees the brake pedal depressed like when starting or doing a burnout, this table is envoked.
Shown below is the Braked Torque Limit table on my 2014 PPV's stock tune. Note that straight from GM, the car will not make more than 169 Ft-lbs of torque regardless of load or throttle position.

I just changed all the numbers to the max number permitted by the ECU.

There is so much left on the table with the factory tune on these cars, at least on my 2014. Just a straight tune to turn off the Torque Management and tweaking the PE (throttle responsiveness, etc) alone without changing the fuel or spark maps completely wakes up this car!