Yes PW:S is effected by +heal.
When an item has increases damage & healing by up to X amount, it means that X is the maximum increase you will get. Other stats do not change this.
The way it works is that you get 100% of X on a spell that takes 3.5 seconds or more to cast (base time, not counting talents that reduce it)
So an item with +50 damage & healing, you would get an additional 50 healing on a Greater Heal even if you have the talents to reduce the cast time to 3 seconds.
If the cast time is less, you just work out
(X / 3.5 )*T = modifier
where X is your +dmg or +heal and T is the cast time of the spell.
So a 1.5 sec cast spell with +50 damage would only net you (50 / 3.5) * 1.5 = 21.4 extra damage.
Instant cast spells are treated as though they have a 1.5 second cast time.
HoT's and DoT's work slightly differently, where you only get the full value of X on if the spell lasts for 15 seconds.
Here is the
WoW-Wiki page on this for more information