PGarnet

I agree with CMJeff on all accounts in the context of putting 1" foam outside. I would like to add to what he says.

The study in AK with REMOTE walls (insulation/vapor barrier on the outside) was based in the hypothesis that if there were nearly double the R value in the outside of the wall creating an exterior vapor barrier, the dew point would occur somewhere in the layers of foam and therefore prevent the condensing of moisture with in the first third of the wall ( the interior ). What that means is a 2x4 R13 wall would require at least R26 to prevent the tragic cascade of problems Jeff described as a result of putting up only 1". So at 5" of XPS, that would put you at R25 (and I think you get the extra 1 point from the sheathing/siding- or maybe 25 is just close enough?). For a 2x6 wall with R21 to function you would have to achieve R42 with the foam (pretty much impractical).
There have been many REMOTE walls constructed here, and I have not seen or heard of their failures when the theory has been properly applied. I wondered the same about the sag problem with the fasteners but have not heard any first or even second hand stories about that.
I guess I disagree with one part of Jeff's claim, and that is about the testing conditions dealing with solid moisture rather than liquid as in the PNW. South East Alaska has a particular problem with not only high amounts of rainfall almost year round, but also winds driving it sideways allowing it to penetrate siding on a regular basis. The REMOTE walls have been tested in this harsh environment and have been reported to perform very well.
Only time will tell if the properly constructed walls continue to function, but I guarantee the "painful redux" will be seen from the improperly applied theory.

Paul