I also shoot with a d800e. I am looking at computers again, my present system built early 2010 is gettting a little slow and bloated. You can easily see if an ssd scratch drive will do anything. Photoshop has an efficiency indicator- bottom left corner. (you may need to turn it on) At 100% photoshop is using ram, anything below 100% it is moving off to the scratch drive. A scratch ssd may help- if this indicator is below 100% a lot- otherwise it will not improve anything.
More ram is always better with photoshop- I presently have 16 and rarely drop below the 100% efficiency goal.
Fast cpu is vital
Make sure photoshop is configured to use as much ram as possible.
Make sure photoshop cache is set correctly for your cpu.
Check the number of history states he is running- more history is more ram use.
Video card and driver up to date- photoshop will use the gpu is properly configured.
This just scratches the surface, but will get you going.
By the way- here is a thread measuring computer photoshop speed, doing a set of filters on a specfic supplied image. This thread goes back several years, but you can compare his present setup speed to other configurations before you spend much money.
NikonCafe.com
Gary