I’ve read the springs for the T-type package are 422 lb/in. for the front and 118 lb/in. for the rear.
The UMI Performance springs are 600 lb/in. for the front and 130 lb/in. for the rear.
It seems the regular regal springs were like 355 lb/in. for the fronts, but I don’t recall reading what the rears were.
Hopefully someone will chime in with specifics on the 1520 springs.