The whole Ron Reeder "use MK for negatives" was been proved wrong a long time ago but people still go along with it. Like Walker said, it doesn't stick to the film and you end up with grainy highlights. That was the case 10 years ago with the x800 printers, and it is still true with the new HD inks.
If you are going to be using the P800 for digital negatives, take a look at my new QuickCurve-DN system. It uses a lot more overlap of the LLK and LK inks and very little PK compared to most other QTR systems. It also includes stand alone linearization tools so you don't need to mess with any of the traditional QTR calibration workflows or Photoshop correction curves.