yeah,as above. Spring rates are the kg rating attached to a spring.
Heavier cars have 'stiffer' springs, to function with the additional load.
When you put springs from a heavier car on, it will make the ride firmer (how much depends on how much the rates differ)
You'll still be able to adjust the height, it just might be a rough ride

maybe there is someone about who is using mk3 coils on their mk2 who can comment.
Didn't mean to scare you!