I've got the SR 3-stop soft and have not noticed any image degradation with my D800. As well, I've taken quite a few shots with the Lee SW150 system for my 14-24 and have been very pleased with the results.
I agree it does seem odd to spend so much on glass and still get great results with a mostly flat piece of plastic in front of the lens.
'There is only one success - to be able to spend your life your own way.' -- Christopher Morley