I did it reverse of you, I ran PHP tunes before I got the 4" exhaust installed, and the tunes weren't made special in any way for accounting for an exhaust.
In my case after installing the 4" exhaust, I feel I may have lost a little bit in mileage for my daily drive to work (fair amount of stop/go in town, ~12 mile drive, possibly due to a loss of a little bit of low end torque when pulling away from every light).
However when haulin' butt on the highway (70-80 mph), I'd say I easily picked up 1-2 mpg over 'before the exhaust' (or at least it pushed up the speed at which I can drive at for decent mileage, as before the exhaust, I used to have to keep it below 70 to stay up around 17.5-18 mpg, and anything over 70 seemed to get me to 15-16 mpg pretty quick).
I'd say you're probably better off putting it on before your trip, but that's based on how my truck responded.
Eric
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2006 F350 King Ranch
Gryphon & PHP tuned FICM
EGR deleted
Bypass coolant and oil filters
4" Turboback
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