Quote: "I have no idea where people came up with the idea that cutting a spring makes 'em stiffer"

I think Rhys corrects you above, but to use a phrasing that helped me understand, a coil spring is a compact version of the torsion bar principle. the shorter the material you are distorting, the firmer the rate.
Since the first and last 1/2 coils tend to go solid you can do some math.
From memory the Volvo 140 coils are 8.8 coils rated, more like about 8.25 in RL (real life). So take that and substract for the end half coils you get 7.25, so if you take off one coil you get 1/7.25th's stiffer a spring. That's about 14%. In practice it usually firms them up more than that.