Your WRX PPP pack does not change the downpipe cat which is also restrictive. It all depends what your goals are. If want a responsive fast road car while keeping the TD04 then I would suggest changing the Downpipe for a Prodrive sports cat, preferably a later one from an 04+ STI PPP which has the integrated heat shield, this will keep the car MOT friendly and produce almost as much power as a full decat. Change the uppipe for a standard classic or STI uppipe with standard heat shields they work well in this set up and can be picked up for buttons. I personally would not fit an uppipe with a flexi joint as they are prone to failure. You have an EGT sensor in your current uppipe which is essentially a cat on fire sensor. This will be removed. It will give a CEL light unless you put a 2.2K ohm resisitor across the plug once the sensor is removed. This fools the ECU by giving a signal which is proportional to a safe operating temp.
As previouslt stated you will need a remap if you decat the uppipe.