The Big issue with the HKS FCD is how it prevents Fuel cut compared to Thor or Greddy
The Design of HKS FCD is poor and badly taught out, therefore your engine is higher risk to detonation and bad things happening
you might as well remove the vacuum hoses from the turbo sensor and fool the computer into thinking it's not making any boost, if you go with HKS and save yourself a few $$
the Greddy controls the Fuel cut a lot more delicate
Why choose Greddy BCC?
The GReddy BCC over the HKS Fuel Cut Defensor (FCD)and the Free Fuel Cut Defensor (FFCD) is simple. The HKS unit reduces the signal coming from the turbo pressure sensor at a fixed percentage, which means that all the data coming from this sensor is corrupt. The FFCD fixes the value coming from the turbo pressure sensor at something close to atmospheric by capping the pressure input to the sensor.The turbo pressure sensor is used for six different functions by the ECU
The Greddy BCC does not alter the signal significantly (I measured a 2.0 millivolt gain) and clamps the signal at a preset voltage that is user adjustable over a fairly wide range. It keeps all the stock functions operating properly, and avoids the sudden shutdown caused by fuel cut when the car exceeds the ECU's maximum boost value. It is well made, and should prove quite reliable over the long haul.
http://www.mkiv.com/techarticles/bcc/why_bcc/Why_the_GReddy_BCC.htm