Even with the VIDI Amiga RealTime 24 on my BPPC060 you only get a new "Live" single frame update around every 3 to 4 seconds... 
My thoughts exactly, the VIDI is probably the way to go. The problem is the I/O bandwidth which has no noticeable performance increase from the 060. The framerate is almost the same on an A500.
If I remember correctly (it's 15 years since I used the VIDI) the framerate increased dramatically if you scale down from 320x256 to, say, 160x127 or something.
Or maybe just doing it the other way around, overlay the Amiga picture (VideoToaster?) on top of the composite image from the camera.
