The side exhaust will net you about the same performance as your stock set up.
An aftermarket h-pipe and cold air induction will probably get you 5-7 hp at the rear wheels.
If you're looking for performance mods, look at a shifter, some 3.73 gears, a new MAF, and some pulleys...
Hint: Someone has some FMS pulleys and an MAF for sale right now in the classifieds. Hmmm, who could that be?
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"The Yellowjacket"
Ricer 1: What the heck was that!?
Ricer 2: I don't know. It looked kind of like a yellow SN95 Mustang.
Ricer 1: Nah, fast SN95 Mustangs don't exist... Must have been a mirage or something...