That's pretty good. So without the video 2x booster that's -13.5% per player. I can't make sense of my notes anymore, but I don't think I was able to get all the bonus in one go.
My reasoning for training in batches is that it seemed when I trained 6 defenders, the bonus wasn't 1.5 times more compared to training 4.
Anyway, it seems if there's a difference it's pretty small. When training in batches you end up watching more videos though.