With a fast CPU, you will be bottlenecked by the customchips. I don't know the exact max speed, but if I am to guess then the theoretical max speed for a CPU writing to COLOR00 is one write every 4 lowres pixels.
Not to leak onto your parade, but if you're contemplating using an A500 with a fat accelerator card and custom-made video capture hardware... perhaps it's time to reconsider your choice of base platform?
