Yes, PPI and DPI really are different. And greyscale PPI indeed relates to a larger-numbered DPI value. I've not previously seen 'equivalence' numbers; where did you get them from? That's curiosity, not doubt, btw.
All that said, I long ago concluded that 300DPI was thoroughly inadequate. I'm OK with a good 1200DPI laser printer, but setting my inkjet to 720DPI instead of 1440DPI gets low enough to bother me (thinking text here, not images). So I really REALLY want that 1200DPI-equivalent eInk display. You know, the one that's made of 100% pure unobtanium, with a bolonium back-plane.
Xenophon