Oh I soooo enjoy not having to do thermodynamics calculations anymore!!!

Basically, ignoring the maths, because the heat exchanger is a fully enclosed system (the hot pipe is surrounded by a steel walled cell, and the hot gas takes a while to travel through the pipe) it doesn't really matter what the heat transmitting qualities of the materials are - they will allow pretty much the same amount to pass. (it would be slightly different if you had a really insulating material but all the likely metals are going to be similar)

What will make the difference is how well the exchangers are designed - the standard VW ones have cast aluminium fins bonded to the pipe, probably the stainless arangement would not be so complex and surface area and air contact is rather important........................