Just as an information: I currently working on a port of grbl to work with the Trinamic TMC26x stepper drivers as provided by these shields: http://www.motioncontrol-community.org/?page_id=36#tos100
In principle these are very powerful stepper driver chips, the only problem is that the Arduinos which the shield is designed for, is probably a bit underpowered. However - that is too early to say - if anyone is not afraid to live on the edge, my branch of the grbl firmware can be found here:
Please note that you need to switch to the “trinamic” branch.