I have been using they gray curve on my last couple test burns and things have been looking better. But i also slowied down the burn more too so maybe that's why it looks better. I will post some pics tonight.
Feedrate will surely effect how light or dark the burn is. There are many variables to fine tune for successful results. Just make sure you keep the profile settings saved for engraving on different types of materials, so you can load them later.
Basemon, I'm not sure what border you refer to, but likely it was done with an image editing program before conversion into gcode. Dithering algorithms are in PEP6 - click on the Edit button to access them.