I read somewhere that 2 inches is the maximum you should lower without changing other parts of the suspension to counter increased bumpsteer. I wouldn't accept thier 2" lowering claim as the gospel truth, though. Sometimes they lower your car significantly more then they claim.
I recently installed an Eibach pro-kit on my 91 LX and it lowered the suspension 2" in the front and a little bit less in the back. 2" was too much, my slightly oversized tires now rub a bit on the inside of the front wheel wells with the steering wheel locked to the side. I haven't noticed big problems with bumpsteer, and the handling difference is like night and day between the eibachs and the wunky 13-year-old stock springs.
Apparently I get a lifetime (or something like that) warranty on the springs such that if they ever sag or break on my otherwise unmodified (hey, how will they find out) 'stang I get a new set for free. As for the other brands I don't know, but the warranty on the eibachs is what sold me.
Have fun getting those front springs in. They were a *****.
