Oh I know Bone is gonna be chiming in on this anytime.
HP depends on the install, your pipes, & a lot of other variables. Gains up to 5-10 HP can be possible but if you throw on a new set of headers along with the right exhaust, well then that would be like the ying to the exhaust's yang wouldn't it?
__________________ When you're going to war, you better have three things if you expect to win: 1. Willpower (One's need to put the petal to the metal); 2. Staying Power (300+ BHP); and 3. Firepower (The wheels of your mustang exploding on the highway like a slug from a .45).
I had my Y pipe area cut off (so I had two pipes from each side) then I had custom bent stainless pipe 2.25 diam formed and welded in. I used two mufflers in the OEM location (need hangers) then had them run the pipe from the muffler back and welded on 22" 2.75 inch polished tips (angle cut like OEM GTs)
cost was 350 but this was back in 01 so... figure 450 today.
may can buy a catback from a GT and have a shop fab it up to the Y pipe so you have true duals for cheaper.
__________________ 2003 Cobra Vert (Redfire) #3938 of 5082 @ 05/27/2003
472rwhp/493rwtq -Modification List - Dyno Sheet 2012 Mustang 3.7L M6 (Kona Blue) 2011 Ford Edge Sport (Red Metallic)