[Porsche-O-Phile]

There's no "one-size-fits-all" answer. You can get very good "R" values with 2x4 framing construction.

You'll get more mileage out of using tight construction methods and holding the builder(s) to good detail construction than fretting over 2x4 versus 2x6.

I haven't calculated it out, but I suspect the cost differential of using 2x6 versus 2x4 construction (assuming there's no other compelling reason to use 2x6 and 2x4 suffices) would more than offset any marginal increase in "R" value via overall envelope performance.

At the end of the day it's more about the detailing than about how thick the insulation is. You can have a lot of insulation, crap detailing and get not-so-great overall envelope performance. By contrast, you can get pretty good "R" values with good/tight detailing and get spectacular envelope performance.