An update for the benefit of anyone else who is thinking of trying this: it turned out my mech hanger was bent. With a new hanger installed shifting is great. One thing to watch out for though: this mech (and other new wide-range Shimano mechs, it looks like) has a cage which is not straight, but bends / tilts slightly outwards, presumably to help with chainline in the 46T. I was really puzzled until I figured that out.
July 17, 2020, 12:47 a.m. - gmtfd
An update for the benefit of anyone else who is thinking of trying this: it turned out my mech hanger was bent. With a new hanger installed shifting is great. One thing to watch out for though: this mech (and other new wide-range Shimano mechs, it looks like) has a cage which is not straight, but bends / tilts slightly outwards, presumably to help with chainline in the 46T. I was really puzzled until I figured that out.