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[Signal 2]

AI [Water (distilled) or water/methanol injection] won't do anything for fuel economy. It's primary purpose is to remove heat from the intake charge and, ultimately, from the engine. Heat is stress. Turbos create alot of heat. Heat is reduced by AI. It also functions to reduce knock and control carbon build-up over time. So it is beneficial even on a near-stock car and stock boost levels. This is the case with my car and I have AI (water only).
You can google terms like Auxillary Injection and probably get alot better explanation than I can give you here.

For a high-flow cat, I recommend BONEZ. All stainless, quality contruction, it accomodates the airpump if you still have it and it flows very well. I've had one on my car for over 9 yrs with no issues.