I am looking at doing the same thing with my 500! The high gloss too me takes away from its all black appearance. I'm very shy of trying new things like Duracoating and such lol...
I saw that you mentioned you want to get a magazine tube extension. With the model of 500 you have, you would need too purchase a model 590 magazine tube and than a 20" model 590 barrel that goes with the 8 round tube. Because the lug nut on the end of the barrel is fixed in position and cannot be removed to allow a higher capacity tube. I have the same dilemma.
This is also an option you may want to consider:
http://www.brownells.com/magazines/shot ... 25124.aspx