For the STI, I've gotten feedback with 80% saying 'yes' to it being louder. For the WRX, depending on the turbo, I've gotten feedback with only 50% saying that they notice a difference. For an STI owner, I would recommend doing this mod as something for a relaxing weekend.
If you decide on doing it, you may or may not hear a difference, but just have fun experimenting.
If you DO hear a difference, please let me know. I'd like to add you to my statistics, so I can better inform people in the future =)