Can Kai'sa's leader effect be used to pay for Deflect's additional cost when targeting a unit with Deflect?

Ruling: Yes, Kai'sa, Daughter of the Void's ability can be used to pay for Deflect's additional cost, as additional costs are still costs that need to be paid to play a spell.

Nuances:

• This applies to any spell targeting a unit with Deflect, not just spells that already have power costs
• Stupefy is a clearer example than Void Seeker since Void Seeker already has a power cost that could be paid with Kai'sa's ability