It's definitely iron all the way through and through, and you notice it right from the get go. I have a couple others but of newer vintage that aren't quite so heavy but still one 20# pot is usually plenty.
Most used are 4 cavity brass or aluminum, mainly since I pour lots of hp's and can roll on with them. The MP and NOE are awesome in that respect. I also use a couple of the Lee 6 cavity and 8 cavity MP's in smaller caliber and know what you mean on the heat. That was one of the main reasons I added a PID controller to the works. I noted what temps worked best and preheating the mold on a hotplate allows me to set it and roll, usually from the first pour. My first foray into making shootable bullets was with a Lee 452-300 RF 6 cavity. After about 5 or 6 dumps it would be frosting them up pretty good and I'd have to let it cool a little. Once I got the PID though, just set it to 725 and get on with it. With the alloy I use pouring HP's I usually run 745 on average with the 4C brass and about 730 with the aluminum as most of them are 2C.
A while back I picked up WC molds in 41, 44, and 45 calibers for my big irons, and haven't even opened the boxes. I figured they might be fun once I retire in a couple of years and if I don't use them the grandsons can fight over em.