I've seen a couple papers (
Here's one indicating that something with concrete on the inside and insulation on the outside with a R-value of about 15 would have a DRE of about 40) on how much less energy a home using a lot of interior thermal mass can use compared to traditional home designs with the same R-value.
Along those lines putting external insulation is a great idea. Going from R10 to almost R40 for a few hundred bucks is a definite win. That said I haven't found anything on the effects of interior insulation on earth-bermed structures. Does anyone have any non-anecdotal information on that? I'm guessing the average in-ground temperature is something like 60F during the winter, although that should swing by about ten degrees.