For Ralliart Airtrek Turbo, I read too many variant. Some said only cosmetic look - bodykit, seats, carpet, rims, grill and airscoop. Some comes with Ralliart exhaust and re-mapped ECU with slightly more torque. But you are right, it's not that different compared to stock Airtreks.
Normal Forrester Turbo vs. Forrester STi, then it's a different story. Hehhee...
But properly tuned 4G63T can be very fast!