Seems like the 2010 Impreza and Outback use the same foglights - the lights, Koito part number 114-77828 however use different housings to fit their respective cars.

For the Impreza, the housing is Koito part number 114-20912 and for the Outback it is 114-20960.

This is handy because there seem to be a lot more Outback foglights for sale than Impreza, if you need to replace one - mine was cracked due to stone damage.

Replacement is simple; remove two plastic fixing securing the undertray under the foglight and bend the tray back out the way. Next undo the three fixings (see photo) and this will release the housing. Disconnect the electrical connector. To remove the light from the housing use long-nosed pliers to pinch together the tabs and push the light away from the housing making sure you release at the other two points (see photo). You'll probably have to adjust where the beam is pointing after refitting it.

I managed to pick up a good spare from Ebay for not much money whereas I've been quoted £210 by a Subaru main dealer just for the foglight.

 

Great info - very useful to know thanks