I already geared my X axis to use an effectively smaller drive belt pulley to get less stepper deflection after finding that I’d killed most of the belt deflection and the stepper was now the major offender. (It’s actually a larger belt pulley to reduce belt strain but with a gearing down from the stepper)
I hoped for a nice round number like 60 steps per mm up from the 40 standard on the machine but it came out more like 66.1XX when I finally got the pulleys selected and did a proper measurement and calibration. Carbide motion doesn’t quite hit the 0.025mm steps any more and sometimes in a job I see X=3.003mm instead of X=3.000mm, I’m not sure I can live with myself having caused these huge errors, perhaps I should move to inches where they’ll all be in fractions instead 
But seriously, once you calibrate the machine you’re unlikely to be on a number of steps per any unit which cleanly divides down in the floating point logic the GRBL is running on, whether you’re in rods, perches, knots & slugs or rational SI units.
On Imperial vs SI, I personally draw the line at 100 meters (109 yards 14 and 2/5th inches) it’s SI < 100m but > 100m I think in miles.