eibach.com:

SPORTLINE KIT
front 1.9 in
rear 1.4 in

PRO-KIT
front 1.3 in
rear 1.2 in

I found these number were very conservative. but in anycase, if you go any lower than the prokit there are 3 things you need to worry about:

1. scrapping/rubbing/bottoming-out
2. vastly decreased shock-life (for the stock dampers)
3. you might need a camber adjustment kit/installation, which can run as much as the springs did.

-w