Peter,
I'm not sure if this answers your question, but a couple of tricks I've used are to up the voxels to 32, and I always make sure the GI anchor is somewhere near the main characters's head or chest (assuming you're doing characters). You have to type in the 32 under GI settings--the slider only goes to 16. You can see the difference if you turn voxel view on. If you're going to include background, then up the range. I *think* the max in the slider, is 1000 but you can type in higher values.
Other tricks are to render your image as .png crazy large (at least this works in Daz with Iray--I haven't tried it here) then use an image editor like PhotoShop to reduce to the size you want with low compression. You'll often get much crisper images that way. Ideally, the first render should be twice or three times what you want the final image to be, size wise.
Don't know if any of that is what you're looking for but thought I'd throw them out there.
:-)
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