Having one part of your body warm will hopefully allow blood to flow to that area, and "warm" the rest of the body...to some exent. I notice that with my decent blackbottoms pants that I don't really require much upper body insulation, and for most of my ride on sunday a short sleeve jersey with a long sleeve jersy over it was fine. I had a light pearl izumi windbreaker and used this when I did an extended downhill, but I took it off at the bottom.
The coverage of the jerseys is what made it all work, and the fact that I was moving at a good pace most of the time and generating a decent amount of body heat. My pants breathe extremely well, but they also insulate well too, so some of the blood flowing through my body was no doubt warmed up to some extent by my pants.
On the other hand, you have to watch some of the extremities like the feet because they loose heat like crazy, when you have a metal cleat a few mms beneath the sole of the foot, connected to a metal pedal, connected to a metal crankarm, etc. The conductivity of heat is pretty large here, and insulation from this will also prevent a huge amount of heat loss. The feet in general also need some insulation all around, just because it's usually more difficult to get a good volume of blood flowing through your feet. Your feet are a lot tougher than other parts of your body, but because of this there's not as much room to pass blood. As of right now, I use some gore-tex socks that go around my normal winter wool socks, but I could always use something better in this area. I'm thinking about some winter shoes for this winter. That's a real good investment if you constantly get cold. It's just too easy to feel like crap and cold because all the heat was sucked out from your feet. It happened in the army when we were standing or sitting in armored personel carriers, just a huge difference between standing on dirt and in one of those. That heat gets sucked right out and you feel like crap overall and cold.
My ride on sunday was probably at or below freezing most of the time, especially in the shade at 7000'.
It usually has to be significantly below freezing before I need a face mask, and it also has to be blowing pretty hard before I'll go for the windbreaker.
It's also very relative to how much heat you produce when riding, and someone who is not pushing very hard will not make as much, so you'll also see some wide variances between people in this regard, no real rules you can use here. I've seen people jogging fully covered up with caps and gloves, when I've been out running in shorts and a T-shirt.
