Surge, honestly I would put money into making it perform instead of dressing it up. The way it is there are pickups that can out run you.
That thing needs a V8, either a supercharger or turbocharger ( i prefer supers) and some good tires.
The money you spent on cosmetic mods would have been better spent on a V8 and part of the cost for a super charger.
EDIT:
I just checked some mustang forums and the two most popular answers for upping a mustang is either:
1: find a totaled v8 at the scrap yard and cannibalize it, it has everything you need, although it may have some parts that you have to replace
2: trade it in for a GT.