I am running 245/45/18.
Stock setup is 255/40/19 Pirelli PZero so I am going from a really stiff sidewall on a 19" wheel to a softer one on an 18" wheel. I can definitely feel the squirm when I do some quick swerves to kind of get a feel for the car. Otherwise it feels fairly good, but in the transitions you can really tell the difference from a summer tire.
I'd say in your case going from the 16" to 18" is really what made the difference in handling. 55 on a 16" rim to a 40/45 on a 18" rim is gonna be a pretty big difference in feel!
Anyways, I'm really happy... I can drive up to the mountains whether it's dry or snowy now.