I was going to write out a suggested testing method on the test server, but I forgot whether the new changes to Corsair and healing have taken place yet and I didn't want to look it up at the moment I wrote the last comment.
Testing method: Go White Mage, equip cure potency gear to cap cure potency, a Twilight Cape, a Korin Obi, and sub Corsair. Go somewhere with Light Weather such as Nyzul Isle. Use Healer's Role (be aware it will only have half potency, refer to this page for numbers). Post results with what roles results, weather and your MND/VIT and cure potency values.
I have no clue if the changes for Healer's Role have been implemented on the test server however.