In the Premium, Sport always firms the suspension, and also changes the transmission programming on the automatic. The traction control is independent, and works in either mode. However, Competitive Mode can only be selected when in Sport, though I think after being placed in Competitive Mode, Sport can be deselected and it still stays in Comp Mode (I am still trying to figure this out). Comp mode doesn't disable all the nannies; it disables the traction control, allows more leeway in Stabilitrak (lets the tail hang out more), and also firms up the steering. It's odd that it's not mentioned in the manual, and also that it requires Sport Mode, as it's much easier to select in the CTS, and they even tell you about it.