To drop a specific height is difficult to accomplish because all springs will lower differently based on many factors like the current load and weight of the car, changes to the suspension, and countless other variables. Many aftermarket lowering springs will drop the car approx. 1-2 inches on stock suspension. You can safely lower the car with stock suspension pieces as long as it is not too excessive. New performance shocks/struts with different spring mounting perches will drop the car even more.
What is more important is the spring rate, rather than drop height. Spring rates determine how much energy is put into and returned from the springs and will have greater impact on handling characteristics than drop height. Excessively "slamming" the car with unmatched springs and shocks/struts will result in unpredictable handling and compromise ride comfort along with extra wear on suspension components. Coil-over systems will give you the most control over spring rate and lowering height at a cost premium. For a strictly street application coil-overs tend to be overkill but many get it anyway for the "cred."
Remember, for some cases, V6 and I4 applications are different. Make sure you are getting the right parts for your car. You can research the different offerings and opinions on a Mazda forum such as Mazda6Club.com and Mazdas247.com
