I bought a couple and swapped the shell on one. In addition to adding sensor action to older Legacy locos, I use them at the end of long freight drags behind locos that have a sensor to trigger an additional crossing whistle (over sensor tracks).
One issue is the sensor car, set to the same engine ID as sensor-equipped loco, becomes the "info" for that engine ID in the Legacy base. Doesn't impact operations, but I would like to see an engine with the same ID as the sensor override the sensor car info coming from the base.
I found that the sensor car, left in program mode, will change it's ID (Eng, ##, Set) but will still trigger the sensor track actions. Convenient for playing around and changing IDs for sensor track playbacks. With show layout operation, I use the sensor cars trigger sounds on trains on different loops.
+1 in a sensor kit. Perhaps offer the trucks, car floor, and electronics so any shell of the same dimensions could be put on top. I would like to see the sensor on aux water tenders for older steamers. Vision cars should a sensor.