Fire mastery gives 7% per point, the synergies 14, better max synergies first

Wrong. Synergies add dmg to the 'base dmg'. Mastery muliplies this new base dmg with mastery.

Its
Basedmg * synergy * mastery

Instead of (that is what you are claiming)
Basedmg * (synergy + mastery)

In the first calculation there is a point where 7% mastery brings more than 14% synergy and vice versa.
It all depends on the actual amount of synergy points and mastery lvl.

You are right i was wrong!
Did some internet research and played with my own skills.
Although it's only a difference of 100damage per fireball, maxing synergies first isnt better dmg