Picked up a few tips on the  Nvidia IRay forum

There is an Iray Performance tips PDF that suggests a couple of tricks to reduces fireflies.

Adjust texture resolutions to the scene requirements.

• Textures compete with geometry for the GPU memory. The higher the resolution of
  textures, the less the memory available for geometry, and vice-versa.
• High frequency textures at high resolutions may become a source of further noise
  and require more time to converge.

The total reflectivity (reflectivy * reflection_color) should be capped to 0.7. It’s OK to use a white reflection color with a reflectivity of 0.7. It’s also OK to use a reflectivity of 1.0 and a reflection color of 0.7.

At blog.irayrender.com/ I found a maxscript filter firefly-removal-max-script that may help.

I’m tempted to design a custom filter in fusion to deal with fireflies in animated sequences. But unless things improve with iray I’m likely to move over to VrayRT for GPU rendering