Andrew,
As you have a J-Spec, full de-cat and an aftermarket back box you will be hitting fuel cut without a FCD. You should be getting around 18psi and as you rightly say, fuel cut is at 14-15. You must have a FCD.
I've never had it myself, but you definately be aware if you are hitting fuel cut, probably a huge jolt through the car as the power is cut - someone on the on this BBS likened it to hitting a wall!
Not entirely sure about this, but I think the FCD is hooked into the ECU, which is under the passenger seat(?).
Have a look at this page -
http://www.trlperformance.com/vfcc.html
I'm currently getting my car de-catted, and even with the stock back box I may occasionally hit fuel cut on cold days.
Hope this was of some use,
Tom.