It would be really helpful to have log files from both of you, as well.
Can you please run the performance test in game, and use the built in feeback that happens afterwards to send us the test results? As well, can you please add ATTEN : C7 (slowdown) to the comments section of that feedback?
I want to help you both, but at this point I simply don't have enough information to know where to focus my efforts.
I'm going to move this topic over the bug / support section as well so I can keep an eye on it and follow up with you.
The particle renderer itself isn't doing anything fancy, it's using Unity's Shuriken Particle system. However we are simulating the particles as physical, and gravity influenced objects. The rendering of the particles themselves should be trivial compared to them being integrated into the gravity simulation. If that's not the case, then there's a fairly serious bug. So I'll look into that for now.
Additionally, can you test running the game with AA (anti-aliasing) in the graphics option set to off?