There is a mana collection rating, but what does this rating do, how does it translate to how much mana is returned per hit?
Here's the exact math on genies for DD. The post comes to the following conclusions:
Genies scale exceedingly poorly and a perfect genie will not pass 350 mana returned.
5,000 damage per swing/shot is more than enough to cap the mana return from a genie, as the base damage goes up, the damage requirement to cap the mana goes down.