Alum has greater thermal conductivity than steel. That is why it is used in heat sinks. The reason an Alum intake feels cooler on the outside is because it has transfered the heat to the air rushing through it. Think about it. Really, this is alot of trouble for negligible gain. At speed, the engine bay will be relatively cool. The reason many companies use Alum is it is cheaper and easier to bend, like someone mentioned earlier.