Well - still fiddling decided while they were unhooked to test the other uses of TOPPAGE so here it is connected to INVERT...
and as expected when hooked to the MODE it does split screen text and graphics...
while watching this I noticed that it was possible to observe the bits flipping in the affected byte more clearly than just watching the character and I think if you watch the video what this means is that the D7,D6 and D5 bits are flipping more often... WARNING - juddering as it is on the A model so graphics frames judder.
https://youtu.be/SvyeijODmYc