The mooney wall is good in some ways, definitely better than stacking on top of existing framing members but you get a small bridge at every cross section and you don't have studs in the up-down direction so if you hang heavy things on the wall you need to screw in at the height of the stud rather than just lining up the width. If you go with 24" centers and space the new section from your existing wall just a little bit(or a bigger gap for even more insulation if you aren't going with batts and use cellulose instead) you could be better off on the thermal bridging front and give yourself more insulation room. I think getting to R40 or close in climate zone 6 or higher makes sense if you can insulate the attic to R60 and mitigate losses through glass through either a framed double sheet plastic shrink wrap or if you need new windows to go with U value .2 or lower. I figure if you are already insulating the walls to go all the way.
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