OK, discharged to 0% (2.000 volts each actually). Raw numbers are below. I'm pretty sure I heard the springs a few times. Maybe I'll capture a time-lapse of this some day. These springs really should have a 3/8" rod between them (not 1/4" like I do). The springs are deforming a bit as a result.
Summary: The top springs expanded by about 1.7mm and the bottom springs by about 2.7mm. The terminals, after I removed the bus bars, were closer together (anywhere from 0.11mm to 0.96mm). My measurements were a little faulty there but it's obvious they got closer.
I think it's pretty clear that the solid bus bars are put under strain in this situation and prevent the compression fixture from evenly compressing (the top didn't compress as much as the bottom).
I will measure again after I recharge the battery.
At 100% SoC
============
Terminals
77.98
78.65
78.20
78.75
77.87
78.14
Springs
26.72 ("top left")
27.11 ("bottom left")
27.12 ("top right")
27.48 ("bottom right")
At 0% SoC
============
Terminals
77.65 (-0.33)
77.69 (-0.96)
77.80 (-0.60)
77.97 (-0.78)
77.51 (-0.36)
78.03 (-0.11)
Springs
28.46 (+1.74)
29.75 (+2.64)
28.78 (+1.66)
30.24 (+2.76)