I just had mine installed for HK$1000. Did that in Intermax in kowloon bay and the springs were purchased from cargym for HK$4400
I think the ride quality has actually improved due to less bodyroll (the boat like handling has also improved a lot!). The spring only stiffens up the ride by very little (i think about 10-15% max) and it wasnt noticed by my wife at all
I highly recommend the ACS to anyone who want a subtle lowering and improvement in handling while retaining the OEM ride quality.
I am running 20 inch wheels with 245/35/ and 275/30 tyres front and rear.