In my experience it depends on the droplet size and location of injection.
Ideally you want a spray as fine as a fog but that is difficult to achieve without high pressure injection.
Pre-compressor injection is used in the industrial world routinely to improve the efficiency of turbine compressors.
It takes very little water to improve the compressor efficiency max of 3% of air mass by weight is the max that gives you any efficiency gain on the compressor itself although your engine may want a bit more.
On my system I did some injection efficiency-compressor and got a small amount of impeller erosion.
On investigation what appears to have been happening is the spray was wetting the inside wall of the intake duct under certain conditions and then beads of liquid water were running down the tube into the spinning impeller as all the erosion was concentrated on the very tips of the impellers, and you could see the liquid path on the surface of the compressor inlet due to oil build up from the PCV system.
The WWII fighter planes injected both the gasoline and the ADI fluid (water injection) directly into the eye of the centrifugal compressor. I know of one person that has done a similar setup with the nozzle located very close to the compressor shaft nut and spraying directly on the end of the shaft. The nut spinning at 100,000 rpm beats the water jet into a micro fine spray moving radially out toward the compressor impellers, and has shown no signs of compressor erosion according to his accounts.
NACA designed a injection impeller that did about the same by drilling very small holes in the impeller hub pointed out toward the impellers and supplied the water to the impeller hub.
In my next setup I will take measures to prevent wall wetting, and have some trip points to re-suspend any water that runs down the intake.
The erosion I mentioned on my compressor impeller had no measurable impact on the compressors performance, and took months of daily driving to occur.
You might find this forum thread interesting reading regarding pre-compressor injection.http://www.aquamist.co.uk/phpBB2/viewtopic.php?t=267