SOLVED!!!! Well, made the problem go away at least, not sure exactly "why" but hopefully this thread now can serve to help someone else out too.
- Took the VSS signal away from the engine controller, and now no more torque (spark, throttle) reduction!

Based on doing a few datalogs of "launches" at different throttle application rates, there seemed to be a strong correlation between when tire spin started and when the torque reduction kicked in. I'm guessing it is some sort of traction control predictive model (vehicle speed, rear wheel speed, increases at an "improbable" rate, tires must be spinning so pull torque). Very strange that a GMPP aftermarket engine controller would have that kind of thing in it... maybe with no serial connection to TCM or EBCM modules that report actual wheel spin, there is some default fall-back scheme in there that GMPP forgot to remove.

Thanks for the help to everyone who posted up ideas!