While I agree that the Castelli Kiss is thinner than the X2, IMHO it's nowhere near as good.
Well, "good" is rather subjective for something like this. I bought the Castelli Body Paint Bib shorts and while the bibs (fabric, fit) themselves are fantastic, the seatpad was too bulky and wide for me and caused chafing.
I recently bought Castelli Body Paint Tri shorts and those have a "Tri Kiss" seatpad that is really thin. I asked Castelli whethe they could make cycling bibs with that seatpad, it seems some people would like that (maybe those people should also contact Castelli).