I took some pressure readings yesterday for another picture of what's going on. These were centerline readings. First, no add-on spoiler:
Next, with the lip spoiler in place at the top of the window:
And finally, with the lip spoiler at the rearmost edge of the stock spoiler:
The pressure readings decreased by 25-30 Pa over the rear window with the roof spoiler, but pressure increased on the roof ahead by 45 Pa.
Pressure on the window with the edge spoiler varied from baseline by + or - 5 Pa, but again the roof pressure was higher by 30 Pa.
I'm going to do some more testing of the rear lip spoiler today between teaching, with more locations on the window and roof.