You heard it here first folks!
I bet the base WRX, at some point, will get a 2.5 turbo. I bet it will be the same (state of tune engine) that the upcoming Legacy GT turbo gets. I bet it happens when the Impreza get redesigned for MY06 or MY07.
With the introduction of the 2.5 turbo STi, it's obvious that SOA will have no problem (IMO) offering WRXs that don't conform to the 2.0 WRC displacement limit. So, a 2.5 WRX, with ~ 250-260 HP seems like the next logical step.
Bob