When you say sputter does it buck you back and forth in your seat or is it just a sound.
Check your A/F ratio if its bouncing around in between ritch and lean you will feel that.
If you cant feel the sputter and the car is hold boost all the way to redline this might sound dumb but it could be your EWG flutter that your hearing/feeling?
EDIT: You said he tuned the car and then the injector started leaking right? I wonder if the injector was leaking the whole time and he tuned it that way.