We have 40F rated down bags and are planning a trip where the average expected minimum temperature is 37F. So we are planning on a freezing night or 2. We may be able to find some kind of accommodation if the temperature were to drop way below normal, but would rather be safe (read -- "warm") than sorry.

We sleep in a single walled tent.

We have silk liners. We have head gear, thermal undies and metal bottles that could be filled with warm water. We also have emergency blankets that can be used next-to-the-skin or as a sheet, over the bag.

(1) We have to purchase new sleeping pads, which brings this specific question: One is rated R 3.2, the other R 5.7. The penalty for more insulation is something like 150 grams (5 ounces) and $50. does it make a meaningful difference? (therm-a-rest claims that it does...)

(2) What about sleeping bags covers or bivies. Said to extend the range by an additional 5-10 degrees F. Hype or not?

Other tips appreciated. Really don't feel like spending a small fortune on warmer bags.