Spraying before the blades is kind of like sand blasting them, over time you will have notiable divets on the impeller along with rust in those divets and a bit of white scale built up around the opening (if not a quality stainless or aluminum), Yes I know a guy who decided to do this and let us check his vehicle this perticular unit was a older DSM (kind of a beater to begin with and he had upgraded turbo to install alread so he really didn't care. So thats where the visual observation comes from.
The turbo still functioned fine with no measurble lose of boost, It just did not look pretty and those little chucks are going some were like into your motor.
Being we actually R&D things, You will see less then a 5 degree temp difference mounting the nozzle pre turbo. Verified by digital temp probes. Not worth the extra wear on the turbo and the tiny little metal chips being sucked into the motor.
Remember water/alcohol cools at the same rate at the same volume.
Edit: to post Found out old boy has been running with no filter for a few months while waiting on his New K&N so once again no solid evidince that post compressor injection harms the impeller. We have coral roads here so thats were the damage likely came from.