PSE Parts
#16
PSE allows you to "control" the valves in the center muffler via the control switch (electrically) at ANY rpm whereas non PSE S cars' cm valves are vacuum operated around 3500 rpm so so, with no direct control. The X51 Kit can be had & installed w/o PSE with no detriment to the performance but as you said, GTS cars come with the added bonus of PSE. PSE cars also have nicer polished tips! Hope this helps.