The drag knife should cut foam core well, and of course with good depth control you’ll be able to do very nice rabbeted joints.
The big thing is toolpaths which allow one to cut corners — the Donek folks have a spreadsheet which is supposed to help with that and dxf2gcode is supposed to support drag knives as an output option.
I would recommend calibrating for belt stretch — it’s important if you’re going to cut things in varying orientations and have them fit (see: Design into 3D: Boxes: Magazine storage for an example).