Excellent stuff. This has been a challenge on loads of current projects so it'd be great to get a fix!
While you're at it - we've found that it might be somewhat related to the photon count needing to be higher on higher-res scenes, or larger-scale scenes, where what you really want is to fire as many caustics photons into the immediate scene. I wonder if you could build in a distance/scale limit, so the vast majority of caustics are calculated in the camera's view, and anything beyond that, or beyond a distance we can control, start to fall off.
And as above, when you go from a 2k test to a full res render, it seems you need to really crank the number of photons to get a good, clear result. This should not be necessary, and would ideally also be fixed.
Cheers!
Lastly, as a PS... I know it's been requested a thousand times, but we really would looooove a simple include/exclude mode for receiving caustics. Or perhaps you could consider some kind of "caustics blocker" material. We actually tried making one, which does work, using a kind of invisible-to-camera approach, but it doesn't fully work, and it ends up darkening inside water and wouldn't work for most use cases. But something like your corona portal that does a similar thing for caustics and controls them not to fire past the object might help if a proper include/exlude method is not possible/feasible.