My signature pic isn't updated. I actually have Red Mage at level 70, and it heals at roughly the equivalent of Cure I. You know, the ability we largely ignore because White Mages never have mana issues anymore. I have kept Tanks alive with Vercure. Certainly more of a struggle, however that's the whole point. I actually have to stop DPSing. On White Mage? Eh, Regen will be good enough.
And yet the most popular MMO ever created plays this precise way with little issue. Kind of pokes some rather large holes in your argument. Regardless, why would heals need to be even weaker than Vercure because of HoTs? Now you're just setting up a strawman. If potencies were reduced across the board to roughly that equivalent, all that changes is healers having to dedicate more time to actually casting heals and worrying over their MP. This doom and gloom scenario simply doesn't exist, or has WoW been a figment of our imaginations?
As Tridus said, you would think healers should focus on healing, not spend 80% of their time casting Stone IV, Broil or Holy.
This only encourages people to wipe and reset the boss. In fact, a primary reason Red Mage's utility is overstated is because once you have three or four raises going out, few groups want to keep going.
And make overhealing cost more MP? Although, I am all for encouraging less overhealing. Chances are, this will make people less inclined to heal...
Absolutely. However, we then run into the problem which prompted this discussion: heals being so strong everything that doesn't borderline OHKO is non-threatening. God Kefka reduces everyone to a 1HP before casting a 20k raid wide. Cure III and Succor will cover it. And this assumes you don't have an Adlo. If said Adlo Crits, you literally don't have to heal at all. What makes First Forsaken scary is being forced to heal the DPS up to full HP. In both Savage encounters the devs used gimmicks that literally demand you overheal because raid wide AoEs just aren't enough. This also why so many mechanics in Savage kill you. They have to, otherwise damage isn't a threat.
I feel potencies can be reduced without going the WoW route. Like I said, there is a middle ground. Currently, you will spend more time DPSing, which kind of defeats the point of being a healer.