These steps from the Z owners manual shows how to adjust launch control RPMs from 3,000 to 5,000. It only works in manual transmission equipped models. With the 9-speed automatic it's all preset.
This happened with me as well. The dealership took the manual out of the car to review it with me, and then I left before they put it back into the car. Fortunately, the dealership (500 miles away, Zs were hard to find back then) shipped it to me.