I played on PTS using Shimakaze with RDF and found it to be extremely strong.
In one battle our team pushed towards a cap which we won and the enemy team Shimakaze was badly hurt but manage to linger away.
I was pushing towards their Montana to launch torps when I saw my RDF pointing to a ship behind an island instead of pointing to the Montana.
Since the Montana was 7-8 km away it was most certainly the other Shimakaze hiding behind an island. None of them had RDF so I was not spotted by them.
I could line up my guns as I went around the Island and could very quickly kill the other Shimakaze before he could react.
Without RDF I wouldn't had known the Shimakaze location and couldn't have lined up my guns perfectly towards him.
In the same game as I was later rushing (west) to the middle cap to recap it and I had an enemy Roon 8km north of me also going west.
Since my RDF was always pointing to him, I know that I didn't had any ships in front of me and could rush without being afraid of their Gering showing up at the cap.
So the RDF is not only showing the closest ship, it also shows what directions are safe.
In another battle while capping, my RDF showed that something was flanking (could only be a DD) and it was trying to attack our BB in the backline. It was easy to follow the RDF pointer, inform the BBs, run back and launch a torp wall when the DD was spotted by me.
There were very few battles where RDF wasn't useful and using RDF felt a little like cheating (wall-hack) and I will have RDF on all my DDs/CAs, not sure about BBs yet.