I'm a chronic overheater, but as a SoCal native I don't function that well below about 10F. I don't have a lot of high-tech clothing but I do know that I'll leak heat mainly from head, hands and feet. For head both my jacket and one-piece have integrated hoods, which in combination with a ski hat do a great job of retaining heat. Furthermore it's simple to flip off the hood to ventilate. For hands I use gloveliners, as it's inevitable that I'll have to get get something from a pocket not reachable with glove or mitts on, and I don't want the bare hand exposed. My son Adam sometimes complains of cold feet, so I got him the Boot Glove, which is a neoprene wrap around the outside of the boot and costs $30. Adam says it's very effective.
However, the coldest temp I've ever skied was -9F at Castle Mt. last year, and there was no wind that day and my clothing as described above worked fine. Worst day in terms of discomfort was January 1987 at Mammoth at -5F with -55F wind chill. My gear was not as good then, so I needed frequent thaw breaks, and I was not able to function in the areas above timberline at all.
