See the comment of The encyclopedia in Sanahtlig's toolbox.
Basically, it increases the healing amount, but not the cap. Heals scale with max HP, you see. But the scaling is so high that you'll hit the cap extremely easily. So effectively, anyone who has a single SSR Eidolon will get zero benefit from Kaiser Dragoon passives as they have enough max HP to hit the cap anyway.
I don't have any Kaiser Dragoons of my own, but that's how the passive started when I experimented with in like like... last June or something.