The NACA reports have some tests on jet engine compressors that show (as I recall) maximum effect on the compressors performance was at about 3% of air flow. Beyond that you may still gain some performance as the water will get carried through the system as if you injected it later, but will lower the thermal effeciency of the intercooler if you have one.
Its very hard to isolate one effect from the other --- bottom line is you keep tuning until performance does not improve, and who cares which one of the effects made the difference
Personally I only ran 2% - 4% of intake air flow on my car and was quite happy with the results, but the turbo was so small I really could not run killer boost levels, so could not investigate (actually had no need to investigate) higher flow rates.
Larry