I had checked the effect of Druid promotions on my ADDA party. There is no change in their hit points and spell points. The only change is in experience point. Changing from Druid to Arch Druid yield 100,000 exp points but changing from Warrior Mage to Honorary Arch Druid only yield 40,000 exp points. However, for both the Arch Druid and the Honorary Arch Druid, getting the Great Druid promotion (after they get the Arch Druid promotion) will yield only 15193 exp points.
In other words, taking the promotion in the wrong order will probably reduces the potential exp points that the druid should be able to get from his Great Druid promotion.