I recommend you use as much of the Min 0/Max 255 PWM range as you can. Theoretically, every PWM voltage change will burn a different shade level. If you limit the PWM range, you will limit the shades that can be burnt. The Feed Rate Change (variable feedrate) feature will expand the shade range even more. Try a setting between 20-25%. If a Max 255 setting burns too dark, increase the Feedrate.
You may need to make these changes to your grbl settings to increase the performance.
It looks like you may have the Pixel Resolution (axis step over) set too high. The Pixel Resolution setting will ensure your laser's burn lines will lay down next to each other without any overlapping or leaving gaps between them. With the 2.8w, you should be around a .006"-.007" Pixel Resolution. You will have to resize your image in the PEP6 image editor using the Auto option. Instructions are on Page 8 in the Tutorial.
I did some editing to your image in the PEP6 editor. I used a Gamma setting of .8 and sharpened it to 20. It will lighten the image slightly and sharpening it will bring out more detail when it's laser engraved.
As far as the white spots, does your laser's PWM modulation wires have an intermittent connection somewhere? What type of material are you lasering on?