What's left to say ...
The DDM Probeam is so good, I recommend it as the first mod.
If you install the HF cat without the GMPP kit, you will get more sound but no lasting performance increase. If you later install the GMPP as Soup stated, the new software parameters will allow the HF cat to increase torque and power. In essence, the ECM will "learn up". To the potential of the cat. I did it that way.