I already tried to use Marlin RC8 with the K8400, the board configuration that they added somehow doesn’t work out of the box. There are some differences in configuration.h compared to the Version 1.5. from birkett at github.
Compiling and uploading worked, but as I startet a print the board does a restart after the first move from a motor. No flaws with 1.5. I also changed the language to German, but onestly, the menu is way better in english.
After that I kept going with 1.5. RC8 was not worth the trouble…
If RC9 will support the UART communication with my TMC2208 Motordriver, that I am using to silent my Vertex, then I will work on it again.
If you going to try it anyway you should use PlattformIO instead of ArduinoIDE, it’s just a better IDE.