Currently heal spells are balanced, when you just start learning/using them.
Cure Light Wounds heals a little bit and is pretty common, Cure Serious Wounds/Cure Critical Wounds are expensive but powerful heals, and Heal (if you manage to find one) is simply amazing, yet power-hungry.
They roughly give *2 heal amount and *2 pp cost for each tier.

However, when you start to acquire character levels and casting levels - balance changes tremendously in favor of Cure Light Wounds.

1) You have more Cure Light Wound spellbooks then higher heals. It usually lead in using Cure Light Wounds as a main heal, and efficiency goes through the roof - it heals a little bit, you need to cast it a lot, and you have a lot of spellbooks to support this.
2) Every heal spell receives +1 to heal for +1 efficiency. This boosts Cure Light Wounds the biggest - you need just +6 Cure Light Wounds to double its heal and +13 Cure Serious Wounds to double their heal.
3) Spell cost reduction is flat too. This is important for Necromancers/Druids/low-level Priests. +10 Cure Light Wounds would cost 3pp and heal 12-19 hp (15.5 average, 5hp/pp) - while +5 Cure Critical Wounds would cost 14pp and heal 13-41 hp (27 average, 2hp/pp).

Everything combined, it usually results in following mid-game/end-game situation considering you find/main one heal - +20 Cure Light Wounds/+15 Cure Serious Wounds/+10 Cure Critical Wounds/+5 Heal (if you managed to find it).
When comparing those spells at those efficiency -
+20 CLW gives 22-29 HP (25.5 avg) for 2pp, giving amazing 12hp/pp healing. It lacks burst heal a little bit, but you have guaranteed potions of ultra/extra healing for this, and burst heal should be used only in critical situations, so those potions should suffice.
+15 CSW gives 19-37 HP (28 avg) for 7pp/5pp, giving 4(6)hp/pp healing. Healing is almost the same as Cure Light Wounds, but it costs ~3 times more to use and you have less spellbooks, so you need to use it carefully.
+10 CCW gives 18-46 HP (32 avg) for 13pp/7pp, giving 2.5(4.5)hp/pp healing. Same as Cure Serious Wounds, just worse. Healing increase is negligible, cost increase is really big.
+5 Heal gives 25-75 HP (50 avg) for 39pp/20pp, giving 1.25(2.5)hp/pp healing. You cant even use it in everyday life, just as burst heal, and potions outshines this skill.

There is paradoxal situation - cheapest and weakest heal spell is the best, because it is cheap, it is common and because of increased healing at high skill values - he is really strong too.

Suggestion:
Change increase of healing from +efficiency to: +0.66*E for CLW, +1*E(unchanged) for CSW, +2*E for CCW, +5*E for Heal.
Additionally increase Heal power from 10d6+10 to 10d6+20 - spellbook of Heal is really rare, and this skill is rarely usable at all, despite being the strongest one of the all.

This would make mid-end game heals looks like this, which seems a lot more balanced and more encouraging to use stronger heals to me -
+20 CLW - 15-22 HP (18.5 avg) for 2pp
+15 CSW - 19-37 HP (28 avg) for 7pp/5pp
+10 CCW - 28-56 HP (42 avg) for 13pp/7pp
+5 Heal - 55-105 HP (80 avg) for 39pp/20pp