If we have no better source, I think we should go with GM
If we have no better source than Google maps, why should I change it from OSM's routings for mainline SS389 and SS389Dir? Which is what we have!
i.e. truncating SS389DirUdd to its new wp 'CorUmbI_E' and reroute SS389 via SS389DirUdd's CorUmbI_E wp at http://www.openstreetmap.org/?lat=40.578438&lon=9.252591.
Looking on Streetview, I remembered why I didn't change the routings (instead just adding the points to allow travels along the shortcut and to better shape the route). While the road marked on OSM as SS389 that we don't have in the browser is signed as a shortcut for traffic going between south and west, the SS389 destinations (there's no road numbers signed at all) are signed through the town - you are
not directed the way GM has, but rather the way we/OSM has, if you are travelling on the SS389.
There's your better source than Google Maps - Google Maps Streetview!