Yes, moving the z axis upward will make the angle steeper, pulling the cutter away from the sides.
Sorry about the photo's ,that was as close as I could get with my camera, download them to your computer and you should be able to zoom in.
My machine cut this pretty good, maybe just 3 or 4 thou into the corner and equal on both sides.so I think the gcode is ok.
your machine looks as though it is cutting more into the x axis than the y axis so you may have some x axis issues.
as far as machine setup goes, my settings are - no backlash comp.,exact stop(CV mode off),everthing else is pretty much default settings
my motors are tuned with the amplifier software, so that woulld not help you.
Remember, once you get your axis set up good you have to change the gcode back to original settings Richard