Not very likely, but still worth checking: I've seen a few WRX's with rear axleshafts slightly popped out at the rear diff. It's not easily visible, since they're usually only .25 inches out or so. But if they are, they are disengaged from the rear diff, which basically would act like a tire spinning excessively. It won't feel like tirespin since the other 3 wheels are catching up over time, and the viscous coupling in the transmission usually heats up over time and reduces this feel. This translates into a slipped clutch feel. And it's not something you can check by just spinning the tire by hand with the car off the ground.
The last time I saw something like this, the owner had tried 3 different clutches, different adjustments, etc. It ended up being the right rear axle popped slightly out. Barely visible to the naked eye. Subaru technical blames this on hard acceleration when cornering- whatever. Just check the axles. Give the stubs a good thump into the diff, if they're properly seated the dust cap should be flush with the housing.