Went back and tested 3 modes now that I feel pretty comfy with the car. Here's my impressions:
PTM Sport 1: seems the safest choice, as it retains both the Stabilitrak and TC, but at an expense of some pace IMO. The car felt a bit restricted on both rotation and exits in comparison to the next 2.
But one can still drive the car very fast in this mode - just proly not as fast the the next two.
Comp Mode (Track mode then press TC once): retains Stabilitrak but shuts off TC - it was faster than the one above and felt a lot less restrictive but one needs to modulate power on exits as 100% is available. The car still felt somewhat restricted on rotation, but overall pace was faster. Note the rear feels looser as you have access to full power with your foot. Since the car has good torque, it can overcome Stabilitrak easily and wag the tail (and/or spin) if you decide to drive it like a hooligan.
This mode is an old "advanced" nanny mode, before GM introduced PTM and is defo more "advanced" than PTM Sport 1 (IMO).
PTM Race - No Stabilitrak but TC remains on, albeit in the least restrictive fashion as compared to Sport 1 or 2. The car rotates in a fantastic fashion and is easily modulated with throttle. But if you over do it there is no Stabilitrak to save the day. Exits feel very composed yet fast, as TC works in concerts with E-LSD in very seamless fashion. This is the fastest mode save for "all off" (albeit GM Engineers will argue Race is faster if used properly - see my OP), but one has to have knowledge and experience regarding proper "weight transfer" principles (balance/traction) as the rear will come around if you don't, or make a mistake and unbalance a car.
No, miracles don't happen and trying to corner on 1 front wheel will always result in a spin
Use at own risk, read the official manual, get proper instruction, etc. Results may vary based on a driver, type of track, etc. When in doubt, turn nannies up: it is no fun going home on a flat bed. Cheers!
EDIT: cleaned up original post