"Doug, if you're still reading this, I'm curious about putting the FF shell over the Nunatak booties; it seems like it would be redundant to me, but obviously you're seeing a benefit. How so?"
For exactly the reason you talk about earlier. It doubles up on the foam midsole, which has two benefits -- better walking around (more solid feel) and less heat loss through.
I use the Chugach booties, though, only when it's real cold. The down booties that come with the shells are pretty warm, just not quite as warm, in my opinion, as the Chugach. And as I said earlier, when it's just 'cool,' I use ID HotSocks with the shells.
And I owned the Chugach booties before I ever bought the FF booties, so I didn't buy the whole thing with all the uses in mind. It just worked out that way!