I believe the valves on the QP S open only above 3000 rpm if the SPORT button is pressed. on a Sport GT S they are open throughout the rev range if the SPORT button is pressed.
From Maserati's brochure on the QP, relating to the Sport GT S:
"This version is fitted with the most powerful engine in the Maserati Quattroporte range. Capable of 440 Hp at 7,000 rpm and a maximum torque of 490 Nm at 4,750 rpm, it raises the car’s performance to unprecedented levels. These figures have been achieved not only by modifying the engine’s electronic control parameters, but also through in-depth development of the racing exhaust system installed on this version. The system is controlled by air valves that open in SPORT mode to reduce back-pressure in the exhaust; this increases the maximum power available, and creates a throaty roar at low engine speeds and full symphonic sound of a genuine sports car at higher rpm."
From the factory, a Sport GT S is noticeably louder than an S - presumably due to the racing exhaust system mentioned above (which as standard was not available on an S)