The key point is that as a MacPherson strut suspension is lowered, the length of
this lever arm increases. Just compare A1 to A2 in the figure
above. Thus even though the car is lower,
it will tend to roll more! This is not what many people expect
when they lower their cars.
There are benefits to lowering your
car - reduced weight transfer and less "jacking". And there are also
disadvantages - increased roll couple, a negative effect on camber curves
and possibly increased bump-steer. Thus, lowering your car involves
a trade off. Lower is often better, but too low is not good.
If you stiffen the front springs at the same time that you lower your car
(this is usually the case) then the stiffer springs offer an increased
roll rate to counter-act the increased roll couple. In this case
you get reduced weight transfer without necesarilly increasing roll.
And often the reduction in weight transfer will offset the reduced camber
curve and bump steer effects enough that the car will ulitmately be faster
around a track. However, you should not lower your car without at
least being aware of the possible negative consequences.