Apparently in 4.0.6 Mastery is getting a 33% buff. Where will this change put the stat in priority? Will it be better than crit? If it is then the spirt/mastery gear looks better than haste/crit gear as above the hit cap you can reforge spirit to haste.
I'm the one that provided the research for the new FoN numbers. Here's a few addendums based on your commentary:
Treants do Physical damage, and armor does mitigate the damage.
They do _not_ inherit the Druid's haste and crit. I believe, but did not have time to test, that they receive raid-wide physical crit and haste auras, as well as Heroism. I am uncertain about the raid-wide +AP% aura.
There should be an entry in the computations for glancing blows. Treant damage is on the white hit table, which has a 24% glancing chance against a raid boss.
To continue this train of thought to its logical conclusion, I tested raid buffs/debuffs for Force of Nature.
Target debuffs, such as the 4% Physical damage debuff on target from a rogue or arms warrior.