I'm ok with all your numbers. Just I miss understood your statement here:
It's fast enough to throw a 320*200 4 plane screen around on a pixel boundary in a pal frame with VBL time left
As it was in the middle of a "sprite" conversation, I though you said you can display a fullscreen sprite ( I mean MASK + OR ) for a complete 320*200 4 bitplans in one VBL. Of course the blitter can't be so fast.