The carnival already glides like a boat on water. Not sure why you'd want the ride to be any softer than it already is. Technically, yes - more sidewall equals a softer ride, but will you notice the half-inch difference? Probably not.
I was looking at TireRack.com for winter tire recommendations and their data says you can go all the way down to 17" and still clear the brakes. I agree though, how much more plush are you wanting this thing to ride?