We have noticed an increase in FPS performance, which is really not a true indication of performance in UE4, as to resources that would benefit from forward rendering in the first place.
Shader complexity seems to be less critical and more so in the area of transparency.
The use of sub surface profiles is out
Landscape materials tends to create missing texture errors requiring they be switched to shared wrap.
Ambient cube maps (HDRI) do not work in FR.
Consideration for use.
If your game has an on line component or plan on using VR any performance bump is desirable and to this point working around some of the feature issues is a matter of finding a different way of doing something.
If your game is stand alone and able to maintain a height performance curve then I would consider to defer the switch until the game is in the final polish stage and can be properly tested and profile as being fit to finish.
Personal opinion as to looks?
Some say FR looks better but I would suggest that it only looks different and neither better or worse.