Did some searching here and did find a post where someone said he'd replaced 5 Banks headers due to cracking. This does not inspire confidence.
That being said....
1. Is cracking common on the Banks 4-banger header?
2. Is cracking related to off-roading? Off-roading for our Jeep will be limited, it is more for getting around in snow and because my daughter always wanted one.
3. People who have the header and exhaust, did it help the mpg and how loud is it? The other thing is for my daughter to go between my house in Pittsburgh, PA and her mother's in Vermont. With a 3-speed automatic and 4.11's it ain't a gas mileage champ.
FWIW, a bud of mine has the full Banks Power Pack setup on his RV and loves it. I've driven it before and after too and the difference in power is incredible plus the mpg is much better, but we're talking about a 32' motorhome with a 454 Chevy.