You need to consider what you want to do with the car. There are different aspects of performance. For example a drag race set up has a different focus from an autocrosser, so you should have a long term plan as to where it is you're wanting to go with the car. The 6 is lighter and has a better weight balance giving it an edge when the focus on braking & handling. For serious straight line performance you'll want to go with a power adder like turbo or supercharger. Engine wise there are lots of little mods like a CAI, underdrive pulleys, performance chip to start you off but don't expect them to result in big power gains. Look to the GT for stock take off parts for a cheap solution for starter upgrades on things like brakes, swaybars, struts, exhaust and rear end. The T-Bird Super coupe is a popular source for an inexpensive route to a supercharger upgrade. Luckily there is now a ton of mod history for your car well documented and on the web. So all you need to do is the research, set a realistic goal and work toward it a little at a time.
'05 4.0: G-Force T-5, MGW Shifter; VMP MAF/ CAI/93oct N2O Tun; CHE K-Brace & Torque Limiters, Rr Ctrl Arm; Shorty Headers; Centerforce Pressure Plate, McLeod DF clutch, Adj Slave Cyl; 8.8" 4:10's w/ T-Lok; Coast Alum D/S; FRPP Dual Ex; 18" Bullitts; CF Hood; Rr Louvers; SS Gen 1 Frt Cover. ZEX N2O - 100 shot bests: 1/4mi ET 13.495/MPH 103.95 (*only mods were VMP N2O tune & CAI; shorties w/ duals; t-lok).