anatomy of Kohler handheld showerhead - flow restrictor
I have a Kohler shower head.  Different model but the flow restrictor assembly is likely the same.  There's actually a series of parts behind the steel retention ring in your photo.  The yellow that you see is the top of a cartridge that contains a spring-loaded nylon stopper (white part in the photo) as well as an orange plastic flow restriction disc.  Remove the retention ring, remove the yellow cartridge.  In mine, there was also a purple part behind the yellow cartridge which is also removable.  Once you've taken all the bits out, you should be able to re-connect the hose and try the pressure again. 

In my mixing valve, I also discovered two 'service valves' which control water flow into the mixing valve.  I found that in my case, the plumber who installed them had them turned to 1/2 and not full.  So my shower head and my rain shower both had very low flow.  I turned the service valves to full open which gave me the flow I expected from my rain shower head.  I then had to put all the flow restriction parts (yellow cartridge) back into my hand shower because the pressure was uncomfortably high.  

Hope this helps!