Have you done something to improve broadphase?
No I havn't, I'm using my own broadphase. I only use Box2D to query fixtures, and do raycasts.
Looking forward to the version finished calculating the forces between particles.
I don't have any plans to do inter-particle collisions in the particle system, but it is easy to extend it to do so.
I see SSE optimizations and spatial hashing for the particles.
Actually, it is just a regular grid, with cells created and destroyed as-needed.
As for the SSE, it is not enabled by default, since it was just some experiments I was doing. In an earlier incarnation of this particle system, I was doing so much aabb testing that it started to show up in my profiling, mostly because of all the branching that took place in the aabb functions. I used SSE to remove most of the branching that occurred, and it did give a speed boost as a result.