yes you should,
see the _HM tile-specific maps might turn out to be in fact ESSENTIAL to terrain, despite what Wrench thinks (as he allways repaints them grey)
we had talk with Nils (Julhelm) a while back and looking at the max scene of ripped SF2 environment Fubar or Russo posted it occured to us the terrains in general are generated procedurally (that is important bit for the future) -a think that I also noticed doing higer resolution (64px vs original 16px _HMs) _HMs for my latest Korea repaint -the higher resolution _HM files generate higher resolution terrain mesh near the camera. So yes, try experimenting with 64x64 _HM files for islands Kferge, I strongly encourage you to do so.
in fact for my new hires tileset I work now on I plan to introduce _HM files for every single tile to check the possible improvements in quality of terrain meshes; all the DEM30 data resolution limitations aside -but that is something I plan to have a closer look at in the future too