On a model locomotive boiler I feel insulation is more a statement of intent than likely to deliver a marvellous improvement! Some materials will be better than others, but they are all too thin to do much good. The insulation on a domestic immersion heater is 2 or 3 inches thick, and that's not fully effective either, even though the water is well below boiling point. Good insulation is bulky.
Always well worth having some insulation, but I wouldn't get too excited about the exact material or thickness beyond more is better! Most materials apart from soft plastics will withstand boiler temperatures, but Cerwool can't go wrong because it's made to get very hot! Can't find any Thermal Conductivity figures for it, but as it depends on trapped air, I guess it's similar to Balsa, which should be fine on a 200psi boiler.
Be interesting to experiment with various materials to see if any are outstanding, but I suspect the thermal performance of all common insulators at model scale thickness will be spoiled by the layer's thinness.
Dave