As my initial personal opinion- I also found the D810 images using the 'comparometer' at Imaging Resource to be notably sharper than the K-5. However, as I understand this tool uses the JPEG image and is therefore influenced by in-camera processing. I also believe that JPEG in-camera processing can potentially be updated sometime in the future by release of firmware if Pentax should choose to do so. And in this case I hope that Pentax may choose to do so.
Since I shoot RAW and process any JPEGs out-of-camera the only fair way for me to compare will be with RAW files. Unfortunately, the Nikon files are not available in a convenient open DNG format
PS- but thank you once again Pentax