1) Disc Hmyn is not a 3 min cd, its 8 mins. (which is precisely why i worded my post that way
)
2) Disc Preist Hymn is the only outlier of the bunch as a legacy hold over. If i recall correctly, casting it is actually a HPS loss compared to PoH Spam during the duration of the channel, so its mostly a HPM gain and the buff gain for other healers and themselves after the channel - though i admit that may be from when it was only hitting 3 targets at a time and not its current incarnation; been a while since i ran the numbers. Regardless its pretty clear Barrier is disc's "raid CD".
3) My point still stands that healers are balanced with their whole kit in mind, ADDING another raid CD as the OP suggested would require either subtracting something from druids (output, longevity, whatever) or buffing the other healers to keep the 6 specs more or less balanced, which while doable is a lot more numbers work for blizzard. - Or the new CD would have to be so weak as to have a very small effect on overall performance (10% is pretty close to that weak point btw)
Now a glyph that turned the shroom heal into say an shroom absorb could certainly be useful.
Glyph of Resilliant Shrooms, Wild mushroom: Bloom now places an absorb for 90% of the heal on each target within range instead of healing them directly. This absorb lasts for 10 secs.
That kind of glyph doesn't shift overall balance but gives you some options for when absorbs would help more than heals (say popping them just before barrage on blackthorn or something) - or my example above for a tranq glyph. Either would give options without just adding more raw power.