I am probably the noobiest of noobs on this site, so take this for what its worth, but I just brewed my 3rd solo batch & 1st recipe of my own design (Oaked Honey wheat) on Jul 4th using a mash tun I made out of a 10 gallon round Igloo and the cheapest plumbing I could get to make the filter tube and spigot. Beer Smith said my OG was supposed to be a 1.052 on a double infusion; I hit a 1.053, but dropped my boil timer at one point and had to reset it based on an estimated time, so I may have over-boiled a bit.
I'm a firm believer in the efficiency of the cooler.
We use the foil bubble insulation in our cockpit sun-shields for the helicopters, and they're not efficient at all. Because of this, I think there would be too much heat lost in a SS Tun even with insulation.