You also really need to take into account that Highmaul is HALF the size of SoO. That is quite a huge difference in terms of ramping up difficulty gradually.
Personally I don't think Highmaul mythic is much harder than HC* SoO (pre nerf) was. Bosses like Norushen and Nazgrim are the equivalent of Karg at/Twins. The difference being instead of slightly more difficult but not crippling fights like Galakras/Shamans, in Highmaul you are instantly hit with a Malkorok equivalent with Bracken/Tectus.
I haven't seen Koragh or Imp Mythic, but a smart bet would be that hey are not far from Siege crafter and Garrosh. I think guys saying Mythic is considerably harder are not really analysing the full picture.