I have been keeping my eyes open for ones like this, I want to run the plasma cutter in the shed which has sketchy wifi at best so need either a remote computer or a reliable comms method.
After playing around with mega 2560 and. grbl, you will not find a ready to go 1.1f .. most versions out there are based on 0.9 .. it seems grbl is mostly left to the smaller 328 and marlin etc for the Mega.
Marlin has very little spindle support... grbl has no proper 4 axis support (I don't think, not looked real hard)
I have not looked into the megapi firmware at all as a mega solution .. i must do that, interested.
Now I am nearing the limits if what I have picked up, info may be incorrect.
Grbl supports arcs, circles etc, so is better for CNC use, marlin is straight lines, small ones but still straight. I think this is an area being looked at but not sure.
If you only have a serial connection to the board then why use a hat, you only have a few wires to connect and probably have a more portable pi and a lot cheaper install.
The 'robot' I am building uses a standard uno with grbl and connects to the pi's serial.
Megapi
https://makeblockshop.eu/products/makeblock-megapi-v1CNC Hat
http://www.ebay.com.au/itm/Raspberry-Pi-CNC-Hat-V2-58-GRBL-Compatable-Uses-Pololu-Stepper-Drivers-/271901344091