Apparently we're closing in on an answer. Or more accurately players on Fenrir (Seriously, Furen was on Fenrir too, what is is about Fenrir and discovering the cure numbers? The forums may never know.).

I tried to dig up more threads but this is all I found:

FFXIAH WHM Forums
FFXIAH General Forums


Kinda sad considering how important these numbers are.

Anyways if we're doing testing made to order can you test this too:
  • Cure Potency and Healing Magic
    WHM 99 Capped Skill, with and without cure potency (24% should suffice however 50% would work better if possible).
    WHM 99 Capped Skill + Merits (although more would be acceptable), with and without cure potency.
    I'm thinking the results of this test will be very predictable but they'd completely destroy any theories for a multiplier, helping to alleviate any lingering doubts.
  • Cure V/VI
    Just need a handful of numbers on this one, mostly to ensure that nothing too scary changed, especially with the threatening brew numbers. MND/VIT being the same would be a plus but not a requirement. I'm mostly interested in the natural cap number personally but extra data is always useful.
    Numbers to test at for skill: 500+, ~450, 424, 301, 232 (Cure V only).