Here is the stiffness of the OE rear lower shock mount in compression:
To know what this means you have to know the maximum damping force is. I did a back of the napkin calculation based on these assumptions:
1) 800 lb corner weight and 1.8 Hz undamped rear ride frequency give an effective spring rate of 45 N/mm (265 lb/in). This translates to the actual spring location as 144 N/mm (by dividing by the MR^2) and is close to my spring rate of 120 N/mm plus the added effects of the rubber suspension bushings.
2) I assumed critical damping, which will over-estimate the damping force.
3) I assumed a velocity of 19 in/sec, simply because this is the highest speed on a typical shock dyno, which presumably has some basis in reality.
These assumptions produce a damping force estimate of not more than 900 lbs.
If all these assumptions have any merit, the deflection in this mount is not likely more than ±0.04" (±1 mm), which to my eye is not much.
I am going to test the stiffness of the upper shock mount as well, as that one is soft enough that you can see it move even at very low suspension velocities as you jack or lower the car. To have a complete picture I wanted to look at both ends of the shock mounting, but the upper end is actually more interesting and easier to modify.