I'm in the process of lowering my car with the Eibach Pro-Kit. There are certain things that I thought were necessary to do this properly. On the front I got some camber plates to adjust the alignment. When you lower it, the tops of the tires come in slightly. It will corner better but may wear your tires funny. My Toyo's are pretty pricey and I don't want to be replacing them early. As far as the rear is concerned there are a few more issues. First off the rear end will shift a bit to the side. This can be corrected with an adjustable panhard bar. The LCA's may not have the proper geometry when you lower it. They should be angled slightly downward toward the rear. I got new BMR LCA's with relocation brackets to solve this problem. You also may have to adjust your pinion angle from your driveshaft into your rear-end (sounds kinda kinky). This can be done with an adjustable UCA. Also you should note that in theory, you should also change your struts and dampners to match your new spring rate. I am actually going to wait till my stock ones go out before replacing them 'cause I haven't decided which ones I'm going to get yet. On a side note, when I was researching suspension geometry I came across an article on lowering springs: it said that lowering a car may actually increase roll rate (which is contrary to what I thought) without taking into consideration the rest of the suspension. They said that you should get stiffer sway bars, front and rear, to compensate for changing your center of gravity or something like that. I am going to wait till I get everything else in and see how she handles before replacing those. Just waiting on a few more parts before changing it over, want to do it all at once.
Rick at radmustangs. com had some pretty good pricing and got my BMR parts from him. Hope I didn't discourage you.
Its between that or a CAI atm.
