Let me preface this with the fact that I dont have forced induction, or water/alocohol injection.
I dont know what exactly the differences would be, but check this out:
Water Injection Kit
The kit was designed around a 1991 Nissan 300zx Twin Turbo pushing 17 psi off the stock turbos. I dont know what changes would need to be made to modify it for a BMW inline 6 like the s50/s52.
Just letting people know another option than AA exists.