I have read the description, and based on what it says, I am reading it like such:
Spell caster is level 15 → Spell is 6th level and does 1d6 every spell caster level → Spell does 15d6 → Add Furious Spell metamagic → Spell adds 2× spell level to damage of spell
Does this mean that Furious Spell
adds the spell's inital level, ×2, to get 12 additional damage on the 15d6? (Example: 6th level spell does 15d6; Furious spell makes it do 15d6+12.)
does it mean that the damage done is essentially 2× (15d6 becomes 30d6)?
I'm assuming it's the former option because 30d6 would be stupid powerful.
In this case, twice the spell’s original level is 12, so it deals 15d6+12.