Some springs are just shortened which gives the lowered look. But they do not have any extra stiffness they should have due to the less height. This normally ends up with the springs coils touching each other.

Some are made the same height but when weight is added (the car) they sit at a lower hieght than std springs. These type springs would be the better type to use.

IMO anything over 25mm drop is too much unless your car and suspension is setup right for it which 90% of cars are NOT!

Regards
Jano