Then you should have stated it that way. Im also a master tech with all my ASE's (useless paper if you ask me.) and A aeronautical engineer along with my A&P. So I know the function/use of o2's.
You said it yourself All the cars at your dearlership use wb o2's. if your running EFI the easiest way to tune is with a wide band and EGT probes.
Why not just build your MS tune without the water injection. and then put it back on. or you could always go Alpha-N which uses RPM, temperature and TPS only.
So the water acts as a baffle? why not just run some type of collector baffle like summit sells. And Have the Chrome headers ceramic coated on the inside.
I own a Coating Company and do Chrome plating, Ceramic and Thermal Coatings. Polishing and Power Coating. I ceramic coat a Ton of Harley Davidson pipes so the chrome doesn't turn blue.
EDIT
: Just as BDMar said above. I was in the middle of typing the above and had to walk away from my laptop for a few minutes.
BTW nice intake in your Av. whos is it?