gasket's purpose is to seal effectively gaps between 2 (flat) surfaces. Skimming the gasket would make it thinner, thus possibly worsening the sealing ability.
BTW, the gasket is already thin and quite fragile. How do you intend to skim it samore?

Heat shield with CAI for my Apexi . http://i3.photobucket.com/albums/y61/Ayok/HScwCAI.jpg