There's a lot more to improving the handling of a road car than just reducing roll rates.
If the car is already stiff enough in roll then making it stiffer still by lowering won't
help,
more likely make it rather worse overall by reducing the effective suspension travel.
Even if said springs are of better quality than the originals, (true, they often are) the
dampers may not be able to cope as well with the new spring rates.
Lowering can also interfere too much with the geometry and introduce new and
unwelcome traits to the handling and steering, depending on the car and to what
extent it's been altered.
Just fitting lowering springs can work, but is certainly not a guarantee of 'improvement' to
handling on many a vehicle as anyone who's driven a poorly-modded car can attest.