Thanks for looking into this!
Now the cache size is not restricted any more but there are still firefly photons appearing when the indirect cache is added and they persist however I change the settings.
The quality of refractive caustics at merge radius 0.25 is just awful, see comparison with BiDir and radius 0.02 below:Note: your caustic merge radius (~0.019) is wrong, just use the default value (0.25).
That would be very welcome, for glossy, metal and the upcoming principled materials.Yes, I could add also glossy nearly-specular materials to the set of materials supported by the cache.
In the image above you see how PGI renders the reflective caustics of the left cube (metal material with fresnel color 0.25 and roughness 0.01) with brute force and produces caustic noise slowly to converge.
In most interior renders there is a shiny floor material made with a glossy material node.
This is slow to render because of brute force caustic noise and also introduces bias because the indirect cache misses the view dependent part of glossy reflections.
Probably with a small (big merge radius) caustic cache for those glossy reflections the caustic noise would be prevented and also the light distribution bias reduced