After exhausting all the possibilities I can think of, and rendering at the highest quality available, and after taking note of some hints from 3D professionals (out of Jeremy Birn's "Digital Lighting and Rendering", I've discovered another more lateral approach ...

Export the Carrara scene to a 3DS file, and import into Vue 6 Infinite to take advantage of the higher quality GI and radiosity lighting there!

That's given me some ideas about how I might improve the Carrara renders.