^^ correct
The tune menu is a bit limited in comparison to GT4, and the methodology is slightly different as well. The S races have Performance Point limits, and the changing of some of the tuning parameters affects the total number of Performance Points of your car. So often, you have to "detune" your car to meet the maximum Performance Points requirement.
I find the system a bit lacking personally, as some of the adjustments that you can make which cause an obvious improvement to the performance, have no effect on your PP total, such as changing gear ratios or spring rates/damper etc, while other things like changing the power, weight, downforce do.