I did some test with HDTV footage, both images (PNG with alpha) and video (QT with alpha) because camera movements are much easier done in AS than in Mirage. No problem, of course the rendering took its time, but screen refresh while working was OK.Do you have any experience with animating hi-res (foreground) bitmaps in AS?
I recommend to limit the number of elements you work with in a scene, like first compose all background levels and render that, then put this as video layer in a file with AS characters moving. (It's a bit like the concept of pre-rendering in After FX.)