Since no one has answered the materials question I'll share a little of what I know.

Plastic is used for two benificial reasons, the first is that in high volume the manifolds can be made real cheap and secondly they are poor conductors of heat and keep the heat of the enigine from heating up the intake air. (Good for TPI) The downside is that it's harder to mod and must be high volume.

Aluminium is lighter and cheaper than cast iron for manufacture, fabricated manifolds are too expensive for production cars. The downside of aluminium for performance is that it conducts heat from the head where it flanges all the way up the intake runners heating the intake air.

Mild steel is much less conductive than aluminium and will heat up the air less but cast steel is expensive and as I already said fabricating manifolds is too expensive for production cars.

Even better is stainless steel as it's even less conductive than mild steel and has a high thermal capacitance which will result in the heat not travelling as far up the runners.

Thinner pipe/ tube walls will reduce the heat conducted from the head up the manifold as will insulative plates and washers on the nuts. On the other hand cold air can increase exhaust emissions and fuel consumption but you're after power right?

Insulating the outside of the intake runners will only help if there is heat radiated from the exhaust or if the intake is cooler than the air in the engine bay and if it's aluminium the intake is likely to be quite a bit warmer than the surrounding air, in which case insulating the outside would only increase air heating inside the intake.

I believe some old v8's had cast iron intake manifolds, there may be others, maybe a ford motor? I think I've seen an old tractor with a cast iron intake manifold to.

Ultimately it depends on what you want to achieve.