Here's what the caustic blur looks like for a mirror-like Cook/Torrance specular next to a diffuse plane, after 1 minute of rendering:
Blur 0 (no cheating):
While doing some more lighting work to the kitchen scene I noticed the performance goes down very fast as I add more light sources, so I improved the logic for picking which light to sample at each path vertex from being random to only considering lights within the domain of the bxdf for the given point and normal. I'm sure there are way better metrics out there but for now it improves the variance a lot, giving me slightly slower render times per progression but a lot less noise per progression with fewer paths wasted.
Here's the kitchen scene before/after the caustic, firefly and light sampling updates:
Rendered at 1920x800.
This one was stopped after 8+ hours with somewhere between 10k and 12k samples per pixel:
And here's what it looks like after the first 16 samples, at 51 seconds: