Shifter formula is 10+1/2 shifter level+wis mod. Owl's insight solves the problem of losing your wisdom bonuses from armor, as long as you have 24 druid levels.

Ahh. I think I see what it might be doing. It's treating it as if you were casting the spell from an item. No spell focuses, and only your wisdom bonus. Fireball is fine if you've got 26 wisdom (10+level 3 spell+8 wisdom bonus), so is combust. The other spells arent following that and are either higher or lower than they should if they're all following the same formula. I wonder if it's just the creature spell versions transfered over to the forms.
I can check if the shifter elemental forms are doing the same with a 20 shifter when I get home in a few hours