Excess is very subjective, though - the Culture's baseline for 'normal' is so much higher than the average IoM human citizen, so its defintion of hedonism would be proportionately higher and less frequent. Slaanesh will definitely get more powerful, but I think Tzeentch would ultimately have the upper hand when it came down to a clash between the two. Nurgle would go first, followed by Khorne (the Culture's style of violence is too clean for him to draw strength from), and finally Slaanesh, leaving Tzeentch Ascendant.
Maybe it's already started planning this outcome in-story...