I flew the A350 a few months ago in Y and was lucky that I only noticed movement going on behind me a few times - probably when reaching in to the seat back pocket.
One thing I tend to do when flying Y long haul (on most airlines) is use the pillow as seat back support. This probably helped minimise minor knocks, but if you have some one large behind you then that's going to be an issue.
I generally loved the A350, the major down fall for me is the lack of toilets on the right hand side of the plane, especially when one was out of action.
Good TR btw.