ModelView's on_change_with() should return rec_name of m2o fields
ModelView's on_change_with() [1] could return rec_name for m2o fields just like on_change() [2] does (although it's not what the comments say [3]).
This would avoid unnecessary requests from the client.
[1] https://hg.tryton.org/trytond/file/tip/trytond/model/modelview.py#l780
[2] https://hg.tryton.org/trytond/file/tip/trytond/model/modelview.py#l817
[3] https://hg.tryton.org/trytond/file/tip/trytond/model/modelview.py#l791