15% Legendary bonus to Spell Critical Damage <- This stacks with profane and artifact. It is "legendary".
Moreover, the standard for legendary spell critical damage is 15%.
Eminence of Autumn, 7 pieces, 15% legendary crit damage
Elder's knowledge, 2 pieces, 15% legendary
Sacrarium set, 3 pieces, 15% legendary
At two pieces, that is a steal. You yourself pointed out bonus stacking on page five, so guessing brain fart.
Yes, it may very well be conceivable to double your spell crit damage. Would have to see where all you can get bonuses. But between gear and epic destinies, this may very well be possible, which would be sick.