Regarding volumetric performance cost, I 've not done my own testing yet but some info from a github commit log was discussed in the volumetric fog thread in the feature requests forum a few weeks back:
Change 3346526 on 2017/03/14 by .Wright
[Copy] Volumetric Fog supports point and spot light shadows
- These lights are injected separately so that per-light resources can be bound (shadow depth map, static shadow depth map)
- Forward lighting of local lights can be forced with ‘r.VolumetricFog.InjectShadowedLightsSeparately 0’
- Shadowed lights come at a cost: 2.9ms for volumetric fog on 970 -> 4.2ms with shadowing