One issue with ft ders that are intended for 50+ T rings and used with much smaller rings is that the cage's lower edges have a fairly large curvature, to best match up with the larger ring's circumference. hen used with smaller rings the ring to cage gap grows significantly as you go from the cage ft tip to the cage tail. This is why I run MtB based ft ders on most of my bikes (that have 44 or 46T large rings).
Unfortunately the imaged adaptor will further locate the cage from the smaller ring's tooth tips allowing for more chain flex and thus likely resulting in more sluggish shifting.
One more cost to having a bike with a brazed on ft der
Andy (who has only built a few frames with braze on ft der mounts and those were when working dor another builder, many many years ago)