I've never heard of this problem with N/A cars, but I don't have very much knowledge of tuning a turbo car. I'd go to
www.rallitek.com and email Sean (I think that's his name). They seem to know their stuff there, as they aren't just selling parts. They do a lot of tuning to Subarus on site. Good guys, and I'm sure they'll be willing to help.
Edit: It might be worth it to come back here and post what they tell you for other people's future reference.