I used Black Satin ceramic coating from www.techlinecoatings.com on my
conversion exhaust.  It is expensive, but shop pricing for a quart is about
$85.  Prep is sand blast, clean with solvent, apply one thin coat with
automotive spray gun, air dry, then exhaust heat cures it.  Different
manufacturers make assorted claims on the heat retention properties of a
ceramic coated exhaust...some as high as 30% reduction in radiated heat.
Tuners tout HP gains made by increasing exhaust gas temperature, which
results in higher flow due to increased molecular excitation.  My brother
worked at Bristol Aerospace and informed me that ceramic coatings are
commonly used in high heat applications.  I didn't make any measurements
before or after of radiated heat, but I can tell you that so far the
coating has held up perfectly to hard use.  The coating also serves to
protect the exhaust from corrosion.  After spending so much time welding up
the exhaust, it seemed appropriate to protect it and also reduce heat
radiated into the engine bay.  Pics at http://my.tbaytel.net/guskers