The editor does not have mesh instancing, hence the massive performance hit when using mesh particles.
However, there's no such hit in neither the Unreal plugin, Unity plugin, nor in the SDK, as they all use mesh instancing.
It's really just the editor that has poor performance with mesh particles.
The feature you mention for the unreal plugin is for Geometry-shader (or compute shader) billboarding, which is not related to mesh particles.
It would be nice to have this, but we're juggling with other higher-priority tasks currently so there's not ETA for that yet.