I thought about that afterwards and hadn't changed things appropriately, but I'm also wary of that. The khorne one i can see for 15pts, but the nurgle champion is already so strong, and a t6 model with fnp would easily boost it to ridiculous, especially with the ability to do that en masse. The tzeentch one is basically what you said, only with melee instead of shooting (for shooting I'd have to increase points cost).
As for noise marines, they get the ability to have sonic weapons. I'd be ok with the champion of slaanesh having a 15pts doom siren option.