We had extended test drives on 20" wheels: one with and one without PASM; having read multiple forum posts stating with 20" wheels you must have PASM we assumed we would have to add to the option list, however after 2 extended weekend test drives the myth (for us) was busted, With PASM we felt that the ride was a little firmer than without and sport mode was too harsh, but better than a no PASM car in sport mode.
Not driven any cars on less than 20's as thats pretty much the default car setup in every dealer we visited, so cant comment on the impact of reduced wheel/aspect ratio, we do have a set of 18" winters, but not fitted them yet to allow us to comment.