I tested the FCD a few years ago working with a Grendel upper and found it had little or no effect on accuracy.
Recently I've been working up a load for a new Stag 3G upper and have found that a very light crimp with the FCD does improve accuracy at 100 yards, similar to what Goldy reported (77 grain SMK, 23.7 grains TAC with matched brass). I have not yet been able to take it out to longer distances to see if accuracy worsens at range.
My experience is for AR use only and my understanding is that neck turning brass is a waste of time for an AR. I can speculate that the FCD makes neck tension more consistent in what otherwise is likely pretty variable neck tension (assuming no neck turning). And considering how the rifle FCD works, it seems reasonable to think that it will make the bullet seating more concentric.
I've also noticed that even a light application of the FCD increases OAL about 0.0015. I'm pulling that number from memory so take it with a grain of salt.