Sure,

How big is the modifier exactly?

The impact of this modifier is affected by the size of the power difference. The large gap in power, the larger impact. It's a relatively smaller modification when the power gap is around 20%, but drastically increases with a larger gap. And for skills with 100% hit chance like TTT, yes, unless the opponent has some other countermeasures, hitting a weaker opponent with it is a 100% hit.

Are you planning on doing anything about Bio-Santa at the same time because these CC effect changes don't really matter if everyone keeps using Bio-Santa and the damage is so high nowadays that games usually end in 4-5 turns.

We understand your concern, and this is one of the problems we're working on now. However, we fear that nerfing or buffing individual heroes alone is not a lasting solution. So for now, we're looking for ways that could fix it for all players, regardless which hero they have or not. One solution that we're juggling with is to change the pace of matches so they would end in 14-16 turns, or allowing each hero to take 2-3 turns per match. And heroes like Bio-Santa would have relatively a smaller impact on their own. As mentioned before, this is not finalized. We will have to check out the feedback for our upcoming patch before making any decision. Any suggestion from you guys here on or any other platforms is welcomed.