Polished????

[zero skillet]

What is meant by polished internals? What advantages does it have? How much does it cost to do it?

[Gastric]

Polishing theoretically reduces friction thus making components move smoother/easier thus requiring less air to operate. However, the reported benefits are just a few psi less operating pressure.

It costs nothing if you do it yourself. There are various directions available if you browse the Upgrade thread.