I still think you are underestimating this. Having to determine where a pitch is going, how fast it is going, and how it will bend, all in a split second, and then putting the bat on the ball is why hitting a baseball is one of the hardest things to do in all of sports. Knowing what pitch is coming takes two of those three variables off the table. Now it is just find it and hit it.
This is why when a pitcher has a tell, and all of a sudden gets rocked until he figures it out is a big deal. They fix the tell and overnight they become a more effective pitcher. No, batters didn't hit 1.000 off of him before he found out his tell, but they sure hit a lot higher off him across the board.
I get that you want to be able to quantify the true effect to determine the real effect, and see if it is material. The problem is that is impossible. Not only does no one have all the data, there are too many variables to ever be able to figure it out. We'll never know, if it is shown that someone got a sign on the pitch they hit a home run on, that they wouldn't have hit the home run had they not known. Likewise, if a signal was given, and a play got out, how would we know if they would have made an out anyway, or possible as you suggest, were distracted and would have got on base otherwise. We'll never know.
What we do know if they cheated to gain an advantage, where there are legal ways to gain that same advantage, and made a concerted effort to spread that advantage around the whole team. That's up to the individual to decide if it is "the worst thing ever to happen in sports" or just worth rooting against the Astros and hating on their fans.