I like to crunch numbers; I'm a retired accountant. But quite a few of the above posts were beyond me, and I will defer to the superior higher mathematical ability of those posters!
My first lightweight down bag was a Marmot Hydrogen (30*). I started getting cold in it, and having to add outer clothing, at 40 degrees. The night it got down to 24*, I had my base layer (Capilene 4) and all my outer clothing on inside it, including rain gear, on a Thermarest LE (2" thick) pad. I lay awake shivering all night. The places that got the coldest were my hips (I'm a side sleeper) and feet, even though to the best of my knowledge I was not stretching the insulation. I made my dog sleep at my feet, which took care of that problem, but not my hips. It was a three-dog night with only one dog! Two conclusions: I am obviously a cold sleeper, and the Marmot Hydrogen, at least the 2005 model, is a bit over-rated.
I sold the Hydrogen and bought a 20* Western Mountaineering Ultralight Super. There wasn't any weight penalty because WM bags come in "short" for us folks under 5'6". I also changed my pad to a 3/4 length POE InsulMat Max Thermo (now replaced by the Ether Thermo) insulated air mattress. The Max Thermo, like the BA, is rated, ostensibly, to 15 degrees. I didn't have a chance to test either bag or pad to close to their limit until a week of frosty nights in Wyoming's Wind Rivers last August. I didn't have a thermometer along, but on at least two nights my dog's water (in a nylon bowl, out in the open) froze completely solid (that was one puzzled dog in the morning!), and there was ice in my Platypus water bottles which were left under a small fir tree with thick low branches.
One thing I found out after my first below-freezing night was that my body's moisture was evidently condensing, freezing and later melting on the outer shell of my sleeping bag, part of it inside the shell. Since I had non-breathable rain gear (the Brawny Gear rain jacket and pants now sold by BPL), I tried wearing that over my base layer as a vapor barrier. It worked--no more problems with moisture in the bag, and I was comfortable, with no excess sweating and no other additional clothing except a fleece balaclava and fleece socks. There were a couple of nights (the same ones in which the dog's water turned solid) during which I woke up cold. However, once I'd made the inevitable (at my age) excursion outside, I moved around enough to get my circulation going and, once back in bed, was able to warm up and go back to sleep. The main problem was when I woke up, very groggy still, and lay there asking myself if I really had to get up! The draft collar on the WM Ultralight Super really helped--tightening that up often solved the problem. There were a couple of times I wanted to grab my jacket, but I never quite got that far--by the time I was alert enough to go that far, I had to get up anyway. I had no problems with my hips getting cold; generally it was my upper body. My last night out, the cold was underneath me, meaning, of course, that the pad was marginal. If it had been cold enough to keep me awake, I'd have grabbed my sit pad, cut from a GG Thinlight 1/8" pad (my dog uses the rest of the pad) and put it under me on top of the air mattress. I will put it there to start with on potentially frosty nights from now on.
I do recommend using the BA Pumphouse rather than blowing up your pad, to keep moisture out. However, while it's a great pump, I found the Pumphouse to be a failure both as a pillow and as a dry bag--it leaks both air and water. I use it as a stuff sack, with a turkey roasting bag inside to keep my sleeping bag dry, and the combo is a little lighter than a dry bag.
I've read, also, that when enough air is let out of an insulated air pad to make it comfortable--I pump mine only about half full--that it degrades the insulation a bit. I don't know if this was the problem or whether the pad would have been as marginal if inflated rock-hard. But I wasn't lying awake shivering for long, so it didn't affect my sleep. Next time, I'll take a thermometer!
It does help to be able to air out a down bag every few days. I was able to do it daily in the Winds. If it's really wet, with lots of condensation in the tent, for days on end, the insulation is going to degrade. Kathleen, I also suspect that after 5-6 months your friend's sleeping bag insulation was a bit dirty, which also degrades the insulation. But you're right; it's lots less expensive to upgrade the pad first!