Here is a new test - a variation on the theme if you will.
Here there are two lights in the scene - one completely inside a glass sphere (simulating a tiffany lamp type effect), and another behind the camera which itself is behind a diffuse plane (so only providing bounce light). The room itself also has a diffuse material.
The ‘tiffany’ lamp is providing most of the light, with the bounce light just being used as a dim fill light. Monkey has a rough glossy material.
No PG
Diffuse PG
RIS
As you can see - RIS again comes out on top.
It appears RIS works well when glossy materials are receiving caustics or light that has passed through a refractive medium first. This is consistent behaviour with my previous scene - the rough glossy material is much cleaner with RIS enabled.