Interesting. Is having 16 GB of ram more important than being able to run the ram at 1800 MHz. I would say the difference between 1600 and 1800 MHz is negligible, so you're better off running all the memory, even though some of it will be downclocked slightly.
With four DIMMs the BIOS will downclock all of the RAM to the default frequency of 1333 Mhz. --- assuming the DIMMs will actually function together as a non-matched set with out issues -- which is a big assumption. You won't be able to OC the RAM with four DIMMs installed.
If you want to run a lot of or large apps like CAD, etc. then more than 8 GB. RAM might be of value. If not then 8 GB. is plenty. OC'ing the RAM makes less than 1% diff in system performance in my testing.