well performance per buck would be a CAI/tuner combo, it will bet you about 20-25 RWHP for around 550 bucks. an exhaust set up, depending on how you do it, (there are countless ways to go about doing exhaust) will net you about 10 RWHP for about 250 bucks for a single muffler to anywhere around 600-700 bucks for a true dual exhaust set up, which i HIGHLY recommend if you decide to exhaust before tuner.
basically its up to preference, a well done exhaust not only looks great and sounds great, but it will give you a moderate performance boost. However a tuner/CAI combo will give you the biggest performance boost for your buck if you only care about that, but im not sure if your restrictive stock muffler will hinder the gains of the cai and tuner
1985 Regatta Blue vert 5.0- 87 5.0 shortblock, T5 transmission, Steeda Tri-ax Shifter, A/C delete, UDP's, edelbrock rpm performer intake, holley 600 4bbl carb, dual flowmaster 44's with o/r H pipe, MSD ignition, 1.72 roller rockers, Tokico HP shocks/struts, FRPP B springs, FRPP 4.10's
2003 F150 Lariat 4x4-DD
2007 V6 Nitrous injected-SOLD
2002 GT 5 speed- SOLD