To be perfectly honest, not too much. The previous owner installed it, and he says it made a world of difference. I still think the heat soaking is a major issue, but to be fair, I never drove the car without **** injection, so I can't reasonably say it doesn't make a difference.
Also for the record, the car is tuned without the **** - its only there when in boost, and was installed to help with heat soaking, not to pick up any HP.