Hi new to the Duet forum, I make CO2 laser cutter so excited to hear duet is moving towards lasers too 
Currently we use the smoothieboard and Smoothie ware,
G0 is used for movements, G1 is used for movements with laser on.
The S value remains as default until a new one is assigned. Same goes for the Feed rate F.
For smoothie many uses don't bother with M3/M5, however we use them as laser enable/disable.
All smoothie movements are dynamic and adjusts the power to reflect the current speed, this helps provide very clean cuts. Most lasers have burnt corners because they are unable to lower the power when decelerating thus do not maintain an even power/mm2. This feature I would say is a must for good vector cutting and engraving.
Example code:
; Pass 0 Path 1
G0 X178.08 Y293.86
G1 X184.19 Y293.86 S1.00 F420
G1 X184.19 Y313.97
G1 X178.08 Y313.97
G1 X178.08 Y293.86
; Pass 0 Path 2
G0 X204.19 Y293.86
G1 X210.29 Y293.86 S0.50 F600
G1 X210.29 Y313.97
G1 X204.19 Y313.97
G1 X204.19 Y293.86
When it comes to raster engraving, . GRBL LPC I think is the fastest currently because it can turn of the dynamic planning. Currently they all work by sending lots of g code, each pulse is a line with a new S value to create the grey scale image. Smoothie begins to jitter at around 80mm/s, GRBL-LPC around 300mm/s.
Kind regards,
Bonne