yeah if it's leaking from the injector you are loosing compression, so will be less power! chances are he changed the washer as they are only cents, but didnt recut the seat inside the head, as when they leak it will wear away a small bit on the seat as only alluminium.
it will now need to have the injector seat checked and the. the head seat as well they are difficult to do them properly and can only be done with the special tool and recut, I've seen many people just put some glue down with the washer or try to tighten the bolt too much, both of which will not work very long or cause more damage! go back complain and ask to make sire they cut the seat and not just try to put another washer on it! best stuff to clean the black gunk that comes out is acetone, I used to do this on a daily basis when I worked for Mercedes!