I had the same problem- My rear end would bottom out over large bumps, and I had my rear spring adjusters cranked all the way down.
I replaced my rear springs with MOOG CC631's (52 bucks from Summitracing)
With the new springs, my adjusters are only turned down 2 inches to achieve the same ride height as before (I still have 3 more inches of adjustment available); and the rear of the car never bottoms out anymore.
Ride quality was also unaffected- My butt felt no difference, except that I no longer have to prepare my spine for major compression every time I go over a raised railroad track...
