I am digging the mia_material document and found this
Use Physically Correct Lights
When GI is enabled, light sources shoot photons. It is imperative for the mia_material (or any other mental ray material) to work properly for the energy of these photons to match the direct light cast by that same light! And since photons model light in a physical manner, decay is "built in".
Hence, when using GI:
Light sources must be emitting photons at the correct energy
The direct light must decay in a physically correct way to match the decay of the photons.
Therefore it is important to make sure the light shader and the photon emission shader of the lights work well together.
So, I could plug a physical light shader into light shader to get a real decay light, but what is a photon emission shader for light? I found none of them in MR material...
Some definition found in http://www.jupiter-jazz.com/wordpress/wp-content/data/tr4kv2/html/chapter1-FG.html
Photon Emission Shader
Photons are emitted from the IBL environment sphere. These photons pick up their energies (or colors) from the IBL texture. A photon emission shader emits all its photons once per frame. It is more active than an environment shader in this sense. Photons work best with mostly diffuse IBL textures.
But can not find a shader for testing right now :(