First off, Heat and Temperature are NOT the same thing....do not get them confused. Heat is directly preportional to the specific heat of the material, so they are only related. Aluminum abrobs more heat, faster, than steel....hense the higher conductivity. It also lets off heat in the same manner. You can put a piece of aluminum foil in the oven and touch it while pulling out your pot pie. Bad example because foil has an extremely high surface area to volume ratio, but you get my point. It takes a bit longer for a steel pipe to get hot.....but once it does, it will be hot for a long while. I can go to my car an hour after driving, and my steel turbo -> IC pipe will still be really hot.
To answer the main question....Aluminum is prefered because it sends and recieves heat well. It absorbs heat from the air traveling within it, and lets it off on the outside surface. It is hard to just look at the numbers and understand....but a higher thermoconductivity means it will get rid of heat at an increased pace. You WANT your pipes to absorb the heat from your air. I also think heat has a tendancy to travel from high to low pressure. This means the direction of heat flow through the thickness of the aluminum pipe will generally be inside surface ---> outside surface. This also means that your aluminum pipe will give off heat from the air within it easier than it accepts heat from the engine and passes it to the air within it. Hope that made sense.
Once I install my HiBoost, I will feel the aluminum pipe after a drive and see if I feel the difference. I estimate that the pipe will feel colder, quicker, than the steel pipes. I can almost quarantee it.
The reason for the thick aluminum pipe in the HiBoost FMIC kit is to "insulate" the air after it is cooled by the Intercooler. This is for efficiency, otherwise, the Intercooler would not help out much. Pre-intercooler it would not make much of a difference, this is why they use steel here.
