John
Great question. I am interested as well.
I refill my 9900 with OEM K3 ink and would love to know the compatibility between the K3 and K3HD and K3 HDX.
We also run a P8000 setup as dye sub and a standard P800.
I have a whole new box of HDX carts that came with the P8000 and would like to try the same same you are contemplating.
By the way the refillables work great in the 9900.
Dan, my understanding is that the 9900, 7900, 9890 and 7890 all use as stated by Epson literature "Epson UltraChrome K3/HDR", so there is implied compatibility with K3HD and the aforementioned printers whereas the newer K3 HDX inks only appear to be branded HDX for the Orange, Green and Violet pigments used in the newer SureColor range of printers.
The recent Aardenburg Conservation Display ratings as I understand it used the HD ink set for the testing not the newest HDX inks, I may be wrong and stand corrected if so.
Brian.