Here's how I would solve the paladin endurance healing problem, without altering Illumination:
1. Replace Sanctified Light, Holy Power, and Divine Illumination with new talents not aimed at saving mana. High level holy talents should encourage the paladin to spend mana, not save it.
Heh, since the priests seem to dislike their 41-pt Holy talent, Circle of Healing, let's swap it with Divine Illumination. That would give healing paladins a significant tool to help them act as main healers in 5-mans. Maybe even reduce its power, drop it down to 31 pts, and move a pumped up Holy Shock to 41 pts.
This change right here removes -11% cost from Holy Light, and -5% cost from Flash of Light, as well as the extra mana savings every couple of minutes from Divine Illumination.
2. For all the +heal gear with +spell crit, replace +spell crit with the equivalent mana/5s. Tthe vast majority of gear like this is paladin set gear, so it shouldn't have very much impact on the other healing classes.
These two changes should be more than enough to keep Illumination in check. Basically, they just reverse the decisions made in patch 1.9, and return paladin healing to the older, more balanced state.