The cap for elemental critical I know is 40%. However, the mage affix aethereal drain and focus both add elemental critical which easily goes over the 40% cap. Does anyone know if these are factored differently and they allow you to go over the normal cap of 40%? If not then that is a lot of wasted affixes that could be used for something else.
No these two give bonuses too the same stat however there are 2 different types of elemental critical affixes the all elemental critical like these offer which caps at 40% and the specific elemental criticals that you can get from epic affixes which have no cap 
Ok, so once I have 40% elemental critical from a combo of 30% legend elemental critical and another 10% elemental critical from the mage talent (or the other ways to achieve 40% elemental critical), I could then add an epic affix like toxic to add another 15% to bring the total up to 55%? Sorry for the long sentence haha.
Yup!