Thanks for the suggestion. I’m running Motion 5 but only have Final Cut Express 4 (since I only work on quick, short projects I didn’t go in for FCPX, maybe next year).
I found Reduce Banding in FCE4 and it does help but not significantly. Curiously the video displayed in FCE4 is quite a bit darker than when played in QuickTime X and the banding is already less noticeable because of that. I suspect the 2 apps use different color spaces or have different ideas about gamma.
But the image displayed within Motion 5, prior to exporting as ProRes, looks really good with very minimal banding. I just can’t get it to export looking that way.