Nah, my Units are set in feet also.  Technically, the radius is supposed to be a measure in millimeters, the amount of penetration and reflectance of your RGB.  This is a separate calculation that is based on its relationship to the Subsurface scale.  Entering a Subsurface scale value of 1 would require you to use lower radius values; decrease the scale, increase the radius.  I normally use a scale value of 0.01-0.02 with radius values set to 0.48, 0.16, 0.109, for example.  It becomes a delicate balance between those two channels.