Changing stereo separation (Unity SteamVR)

Is it possible to change the eye separation distance with the SteamVR plugin in Unity? Changing the stereoSeparation property of the Camera component does not seem to have any effect.

I had to scale down the VR camera in Unity to have the head tracking match the size of our game world, but this also scales down the eye separation distance. This completely removes the depth perception that 3D vision adds.

I'm using the SteamVR v1.2.1 plugin on Unity 5.5.