not sure about the gains, but keep in mind that with the off road pipe you will need either a custom burnt chip or an o2 cheater to keep the computer from throwing CEL all the time. I do know where you can get a brand new o2 cheater cheap if you are quick.
HINT: Check my sig.
Good luck with the off road pipe. I'm sure some one else will know about the gains.
So, would I be better off getting the off-road pipe or a shorty header. I don't want the exhaust too loud but I also want a noticeably diferent exhaust note. I'm sure most everyone here has heard the different sound a car produces when it has a catback and then a catback with a header.
I have the jba short head with a borla cat back.... it's got a great idle, kind of loaps (I also have a supercharger though). And a cruise it actually stays relatively quiet. On my foci you REALLY notice it under accelaration though.