White Mage isn't going to be the best at using Regen spells but still will be the best healer. I don't think there is really much more to ask for then this.
Currently Cure V can get something to the tune of ~16.5 HP/MP (counting cureskin, ~22.25 HP/MP). Compare this to Regen IV's "best" (basically devoting merits and gear slots to it) ~13.85 HP/MP.
Easiest way I could think of currently without affecting "balance" too much would be to increase duration based on Enhancing Magic Skill. Scholars could still cure the most with a Regen spell but it would last longer, meaning more MP efficient. Of course, this would mean that using a Regen spell, Scholar would be the most MP efficient healer. Orison Mitts +2 would be changed to just boost the duration by 25% (basically the same as now if the duration is the same). Enhancing Magic over 300 could increase the duration by one tic every ten points to a maximum of 20 extra tics (at 500 Enhancing Magic). This would effectively double the duration, meaning that a White Mage just gearing for Enhancing would get ~16.22 HP/MP Regen IVs, while one gearing for Regen could get ~27.7 HP/MP Regen IVs. This would make the spells make sense to cast again from an efficiency perspective, however it would make the version Scholar's get incredibly MP efficient, given their buffs (and Red Mages too for that matter). As a result, I'd suggest that the duration bonus Light Arts gives does not stack with the skill boosts (however a Scholar with capped Enhancing Magic skill/gear could get a duration boost since it would be just slightly longer). Basically, while a White Mage could get two and a half minute Regens, Red Mages and Scholars using Stratagems could get something like seven and a half minute Regens.
All that said, if I was SE, this would be a very low priority change, since something like this isn't really going to change much. If anything, such a change would happen with Red Mage getting another tier or two of Regen, as a buff for them. If SE was going to devote time to adjusting White Mage's healing in particular (basically, not the cure formula changes they said they were mulling over that would benefit everyone), I'd hope it would be spend either on fixing Cure VI or buffing Afflatus Misery, not fixing the Regen spells. If they decide to change the Regen spells, more power to you, but I wouldn't hold my breath.