I wrote a fairly lengthy detailed reply to you and then somehow deleted it, so I'll try to recreate my response now. In general, I would say that this issue is more about an interpretation? of how moxabustion effects the body than an issue of metal heat transfer. Personally I feel that moxa transfers the energy of the herb into the body by generally heating an area and then both the vibration of the herbs along with the chemical activity from the smoke create the changes internally. Warming needle in my mind is more of a way of directing this energy and/or combining what would normally happen with say a cone moxa with the effect of the needle at the point together. In other words, the needle isn't really "warmed" - it's just a way combining these two techniques to bring about a particular change. Long story short, then, I don't believe the heat transfer rate of the metal would have any bearing what so ever on the clinical effects of the moxabustion.