On stock dampers I wouldn't go any lower than -40mm F/R, anymore and you risk them leaking and failing over a short period of time.
With uprated dampers like Koni, you'll manage 60+ at the back, but the fronts might need the linings cutting away if you need similar at the front.
Bout as low as the fronts will go without scrubbing with a passenger. Lowered on Konigsport coilovers.