I use both a grbl-Mega and a grbl-LPC on my raster laser engravers as they allow me to run my 7.5W and 40W lasers at much higher speeds than the standard grbl can handle. I have noticed that LaserGRBL does not utilized the full serial receive buffer on the mega or the LPC. LaserGRBL appears to be fixed at the 128 bytes available on the 328p. Grbl-Mega has a serial buffer of 256, and my Grbl-LPC has a massive 10240 byte buffer. Can you add either a feature to manually set the buffer size LaserGRBL will use, or have LaserGRBL read the buffer if $10 is set to 2 or 3.
With $10 set to 2 or 3, the return status from '?' with show the internal buffer state. On startup then the buffer is empty, it will show something like this:

?
<Idle|WPos:0.000,0.000,0.000,0.000|Bf:249,10240|FS:0,0|Pn:PXYZA>
ok

Bf shows the planner buffer and serial buffer available.

Grbl-LPC with it 250 planner buffer and 10240 serial buffer allows me to run the most complex dithered rasters at feeds up to F15000 without stuttering.