Are you doing work yourself, or do you pay someone to do it for you? A supercharger would be nice, but what is the extra power without traction? If you decide on spending the 3k on your current car instead of eventually getting a hardtop, I suggest doing suspension work. Lower it about 1.5" (I have the H&R SS springs and love them), get new shocks and struts all around, caster camber plates, new ball joints and bump steer kit, new bushings all around, lower control arms, upper control arms, thicker sway bars, and definitely subframe connectors! If money allows it, maybe add some cams and a dyno tune as well. A little thing that might help airflow would be a bigger plenum, since you already have a bigger throttle body and a CAI.
2003 Mustang GT
Intake & Exhaust: K&N FIPK CAI, Mac O/R H-Pipe, Custom Catback w/ Magnaflow Magnapack Mufflers.
Suspension: H&R Super Sport Springs, KONI STR.T Shocks & Struts, J&M Caster Camber Plates, Steeda Bumpsteer Kit, UPR Elite LCA's & UCA's.
Performance: BAMA 93R Custom Tune, FRPP 4.10 Gears w/ Terminator spec T-Lok.
Planned Mods: FLSFC, Eibach Sway Bars, Throttle Body, Plenum, Rims.