How does lowering a car affect it?

In my personal expierience loweing by 30mm does not make a great deal of difference. Obviously sports springs are alot firmer than stock springs so ride quality will be affected, the car will usually handle a wee bit better. The only thing you will need to keep an eye on is your stock springs (Presuming you are using stock ones) as these will be constantly compressed 30mm more than usual which can lead to leaks. If you havnt got the springs yet you should really go for coilovers. Atleast then the components will be designed to work together and you can adjust to your liking.