Spirit of Xylem’s 10-20% Reduced Spell cost is only applying to Frost Tempest Strike
Tempest Strike Mana costs with 4/4 Ancestral Swiftness (-4 flat) and 18% Spirit of Xylem
Frost Tempest Strike - 6.6 mana
Wind Tempest Strike - 8.0 mana
Thunder Tempest Strike - 8.0 mana