There's a lot more to this than simply the heat conduction and convection through the pipes.
Stainless and Aluminium pipes can be much more highly polished which can SIGNIFICANTLY reduce heat gain due to friction of the air passing over the surface of the pipe, and if you think that this friction is nothing to worry about, you would be quite wrong. The rate that the air passes through these pipes is VERY fast, and the faster you go, the faster the air flow is through the pipes, which means greater and greater heat gain due to friction.
In a stainless or aluminium pipe, these frictional effects would be mostly transferred to the pipe, and then convection heat transfer would remove the heat energy from the pipe to the air blowing under your bonnet.
So, with a plastic pipe, not only is your friction effects greater, but the friction energy is transferred more to the air inside the pipe than to the pipe itself since the pipe does not conduct heat as readily as metals.
There is a method in the madness.