This first test is Fury
20 million nParticles
motion blur switched ON
4 x multisampling
13.26 seconds (dual Xeon E5 - 32 cores, 48GB RAM, Nvidia Quadro 4000)
I'm showing the alpha channel only because I currently just have the demo version of Fury and the watermark is distracting in the colour channel.
The next test is Krakatoa
motion blur OFF, render time 11 seconds
It's slightly trickier to get started with Krakatoa, but I think the results look amazing. Again, this is a lot of particles (14 million)
Here is my setup for Arnold, but I cannot seem to get the opacity to work properly.