The WRX (which is now its own model instead of an Impreza trim level) has a different engine than the stock Impreza.

The Impreza WRX was the rally platform, kind of like how you can get a fast Civic Type R or a Golf GTI as opposed to the "regular" model. In recent years, the WRX has been spun off into a separate line, dropping the Impreza name. The regular Impreza is separate.

When the chassis were lighter it was a better combo. Now the chassis are so heavy it's underpowered.