On the rears, take out the arch liners and you can go very low! i had custom 100mm springs on the rear with no problems, then raised it up because it was a little too low!
on the front, the first thing to hit is the arch liners......
take out the arch liners and you can go lower until the shaft hit on the inner wing.....
cut and weld the lip on the inner wing and you can go lower, until the ARB hits on the subframe due to the angle of the wishbones......
(this is where i stopped)
you CAN remove the ARB to go even lower, but i never went this far, and youd soon find the shafts hitting the chassis rails again