Spell Focus definitely has effect on all spells cast regardless of class.
Conjuration focus is also one of the best ideas for a druid to take, so definitely not a waste.
The DC is easy to verify, you know your base, spell level, wisdom modifiers and focii, so add them up and compare with the DC used in game.
So: DC = 10 + spell level + ability modifier + feat modifier (+4 for Greater Focus)