It's not the supercharger that makes the GTP more efficient (well, not like a turbo would anyways), its the lower gear ratio, so you run at lower RPMs for a given speed. 2.93 versus 3.29.
For handling, make sure you upgrade to the GMPP handling kit.
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'99 GTP, L36 bottom, XP cam, Headers, 3.29 gears, FWI, Powrtuner