I've got a couple of Deathrun maps with particle problems. On one of them, I've got some env_sparks and on my computer I don't see them, but other people do. It's a very simple map, just a corridor. No special effects. Then in another map I have some env_steams. I set them to reasonable parameters, even ones which are actually better than the previous version of the map but the steam is really patchy when it comes out. In that map I do have other things in the map that use particles, but they aren't drawn at the same time and I even hid them and compiled without them, and I still have problems.
I've also noticed that I sometimes don't see a func_smokevolume in spawn, but sometimes I do. It might be to do with the fact Hammer launches its own little version of the game that has no anti-aliasing, and maybe some other graphical reductions, and I can't see it in that but I haven't checked.
With regards to dustmotes, if you use a render distance with them so only a handful of volumes are drawn at once it shouldn't matter how many you have, should it? Dustmotes don't strike me as expensive seeing as they're just dots going up or down.