What sort of framerates can you capture at Banshee?
This is a good question but to be honest it soo much depends upon the game. I use delta timing so if my game slows down it compensates and copes, on the two streams i've released on the net of my games i've produced final movies at 15fps to keep download size down so I guess you could lock your fame to an fps rate and achieve better results.
I hear that running in 16bit mode instead of 32bit also speeds things up.
It depends entirely upon your graphics card, some more modern cards actually run 32bit faster than they can 16bit. There's no rule to this that i'm aware of, but if it is shader 2 equipped stick to 32bit, if it's shader 1 then either mode may achieve better results.
is it practical to save straight out of Blitz somehow?
Yes perhaps. It depends how you have done your timing code but there is nothing to stop you locking the "framerate" of the game to whatever final fps you want your movie to run at - whether it plays at that speed whilst you export the movie or not - that way you could just save out a series of images and import them into a movie editor.