Diffuse Boost not working?

HI there. I’m having issues getting any ‘influence’ out of the Diffuse Boost function in the Material editor.

So I have an environmental scene with a number of artworks throughout. I would like these artworks to contribute more ‘light bounce’ via the lightmass global illumination. From my understanding Diffuse Boost is meant to do this, however, no matter how much I crank it up, I see absolutely no difference. Now I could use ‘emissive’ however I don’t actually want the works themselves to be bringing ingame.

Am I missing the understanding of ‘diffuse boost’ ? Here’s a video link to the project in question. Thanks