Probably about 3FWHP on a moderately modified SVT over just replacing the stock resonator and rear mufflers with straight through.
The 2.25" single is a restriction especially in the upper rpm (above 5k)
Cost analysis.
2 mufflers and a resonator is ~$150. Probably $250 installed???
Trubendz is $400 IIRC. Install it yourself. Maybe $50 to have a shop weld it up once you install it. (clamps SUCK!)
Even though this shows how cost effective modifying the stock exhaust is if you plan more mods or "significant" mods then the 2.5" is the way to go.
I only recommend duals to those that can build their own.
I'd imagine for a shop to build a good setup (Mandrels and straight throughs) it would be over $1000 and you'd have to either find a shop that is highly skilled and trustworthy or watch them like a hawk. The later of which just won't happen.