and its the actual particle update portion that makes the hit to performance. Having to read back from the GPU every frame to keep particles in sync is really rough

You read the particles from gpu? Try not to do that, also, I doubt too that dynamic buffer will outperform geometry shader.

http://www.slideshare.net/icastano/cascades-demo-secrets

From page 119.

Nvidia Cascades demo where they are running full gpu particle system. Particle updating happen on vertex shader, emitting/killing particle happen on geometry shader. They use double buffered vbo and stream out. Super simple stuff and general enough for most thing. Speed should not be problem with those particle counts.