The composited output maybe 16bit, but my banding problem is definitely gone. It isn't directly related to 16 bit output, but to loading textures as 16bit, and that kills the alpha channel

just have a look at the blue nebula in the background:
http://dl.dropbox.com/u/6050252/16bitQGLN900.png
http://dl.dropbox.com/u/6050252/32bitQGLN900.png

So yes it did solve my problem