Extra fine steel wool and acetone (wear proper gloves; butyl rubber
, not nitrile) will clean the pipe up like new.
I recently "restored" the headers on my old Royal Enfield 750 which had 20 years worth of old primary case oil burned onto them with the above materials.
They're nice and shiny now.
I thought those Dowco covers had metalized fabric on the lower half where they're likely to touch the exhaust system?