Thanks for the helpful replies.
Jeffrey: I agree that I will be warm enough in the 15 degree bag at night, and I have the Hoody and/or Hadron to supplement if needed.
Terry: Good point about experimenting. I have to remember this is an adventure and being "wrong" is a minor issue in this case. Unfortunately I don't like wool (even Icebreaker), as I have very sensitive skin and it bothers me. (When I tried on the Icebreaker and said it was scratchy, the salesperson freaked out :))
Steve: I will have a light synthetic T, so the Cap 4 will be a mid-layer for this trip.
All of the replies made think more about this, and I now realize that I really want a fresh set of light clothes to sleep in and I think the Cap 4 stuff may be too warm. I am going to think about it a bit more, but my tendency at this point is to (a) replace the Cap 4 long johns with silk (I have cheapie convertible pants (12 oz.) and rain pants (7 oz.) and doubt I will really need more warmth than those plus silk around camp), and (b) keep the Cap 4 hoody and supplement with a silk long sleeve top just for sleeping.
The weight difference should be negligible as exchanging the Cap 4 long johns for the silk long johns should just about offset the added silk long sleeve top.
The only negative is two pieces of silk underwear are pretty expensive. Has anyone tried the Terramar brand they sell on Sierra Trading Post? How does it compare to the REI stuff that seems to get good reviews?