The central adjuster adjusts the angle of the internal torsion leaves in the beam adjusting the height up or down depending on the position of the adjuster.
Adjusters rely on torsion leaves, you replace these with a round bar when you fit the red9 coil over and all of the adjusting is done on the coil over. You should be able to go down just as low. If you want to go lower you will have to fit dropped spindles. there would be no point in fitting adjusters but if you have them already fitted you can still fit the red9 coilover kit.