Posted for Walter Atkinson <walter@advancedpilot.com>:

 Coating on the INSIDE is a good idea.  But, one could be putting a turbo at
risk if parts of the coating comes loose and the hard ceramic hits the turbo
fans.  There are many possible benefits of coating the exhaust, but I wouldn't
want to overlook the possible downsides.  For ME, the benefits don't outweigh
the possible downsides.  YMMV.
 
 Walter