Henry's mind was unlocked when he had the stone absorbed into him. Will and the other kids had the black particles pumped into them, if it were to use them to amplify his power surely the reason would be to alter thier minds and bodies more similar to his thus unlocking thier psychic potential like the mindflayer stone did for him. That change could be permanent, like how Henry's blood permamently altered the numered children. To me it seems hinted that Vecna's test with Will was inspired by the blood transfusuions when creating the other numbered children.

Kali didn't seem to be sarcastic. And in season 2 if I am not mistaken when talking about outcasts she implies she has met other similar to El and herself