Hi - Thanks for taking an interest. I have heard and see your point of view, but there seems to be conflicting stories. For example, on the 912 Registry site (great site Rick) at:
http://www.912registry.org/tech/exhaust.htm
Exhaust and heating systems are discussed and two 912er's say they use J tubes with cooler results. I've also read about using a VERY thin coat of high temp black paint.
If the pipes are surface rusty, would rust convertor (changes rust to a polymer, looks black) be good or would the change actually retain more heat?