Depends. So 12/6 is 12kg/mm front, 6kg/mm rear. 1kg/mm = about 56lb/in, so 12/6 = 672/336. They basically just cranked the spring rates through the roof and didn't bother to change the balance any. If you like the balance of the car now and just want stiffer/lower suspension, these will work fine. To me, the front is a bit too stiff, and the rear is too soft. My BC coilovers are 8/7 front/rear- my rear rate is a lot stiffer compared to the front than this CX Racing set. Even with the higher rear rates and the monster sway bar I added in the rear PLUS my aggressive alignment, the 5 still understeers a bit- but it's much improved vs stock. Given how stiff your front springs are, your ride may feel a bit unbalanced, but if the shocks are matched up to the springs (I'm sure you'll be able to adjust them to the right setting) then you'll still have a good ride. You might want to ask the seller what all the extra parts are- the black cylindrical things on the very right side in the first two pictures have me a bit puzzled.
FYI- you generally make the rear suspension on a car stiffer in proportion to the front to get it to turn better / more responsively. All car manufacturers tune their street car suspensions to understeer so that the car buyer won't get into trouble as easily. Since the 5 is essentially a family car, you have to mess with the factory balance a LOT to get it to dance more easily when you smack the steering wheel around. A LOT.
