The CP Plug stops you accidentally turning the air supply off
The DM9 Eye Pipe is pretty much a must have
The DM9 trigger also fits, and offers rake adjustability over the stock trigger, or there are other 3rd party triggers (CP/Critical etc.) if you prefer a different shape/style
Laser eyes, as above, not really worth anything and often less reliable than the stock ones
3rd party boards offer little actual advantage and often bring reliability issued, the std UL Boads ar pretty good
A decent barrel kit (CP for one) with a choice of bore sizes helps increase accuracy and efficiency by allowing you to match paint to bore, also you may prefer a different length front
HTH