I'm not sure I am at all entitled to give advice on the subject, but I'm going to try and offer what I can:

For the cone, what I found to be most useful is to turn 3d shadows on and to increase falloff to about 2-7% (in post cases) depending on the scene. I would also render it on a separate layer and composit it adding some blur, that allows you to get away with minimal increase in rendertime. Otherwise you'd have to up the quality of the cone, which, with GI already ON can get pretty expensive time-wise.

For the dust I used volumetric clouds before (turning on turbulance), but I can't say that I'm entirely satisfied with the results. But if you want to go that way, I found that the coulds included in Howie Farke's Lake scene can be a good starting point, but you can also just play with the settings.

Here is an image I did using the cone (but no dust):
http://antarann.deviantart.com/gallery/#/d4ehita