Sometimes in turns the AWD hunts then digs in a little.

It's like if I'm uncertain it will be uncertain too.

Most time it's quite rational Now that I think of it could be a warming issue.

Still a steering angle censor would be all that is needed.

I don't think the 06 impreza has it?

I couldn't say exactly when, but generally if your car has an advanced stability (ESP, ASC, ASR, ESC, etc) control system, it will have a steering angle sensor. Regular traction control systems don't take into account steering input, just wheel speed. The more advanced systems actually effect vehicle direction control rather than just wheel spin, so they use steering angle, throttle angle, wheel speed, vehicle speed, G forces, and tons of other values to determine which wheel to apply brakes on, how much to change the throttle, and if it needs to adjust steering sensitivity or any number of other various systems of the car to restore stability of the car.