I can't seem to find this basic bit of info anywhere in the afconvert literature. Does the true vbr AAC mode (maximum quality, i.e. "127") in afconvert/QT scale all the way up to 320 kbps if deemed necessary? If not, does anyone know what the maximum upper limit bitrate for this mode is? I'm assuming that the displayed bitrate in QuickTime/iTunes is an average calculation of the entire file (obviously).

You'd think that the afconvert manual would explained this in more detail, but I've yet to encounter anything definitive.

Q127 = ~192kbit/s.

It's also not too efficient of a codec as of yet. See here.

I thought so. Thanks for confirming this. I think I'll stay with VBR Constrained @ 256 kbps.