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So far as making any judgement about the condition of your engine is concerned, there isn't really an OK or not-OK amount of oil to have in the throttle body like in your picture, as it almost certainly won't be coming from a leak. Much more likely is that it's just being sucked in through the air intake pipework from the crank and/or cam breathers, which is completely normal on a standard Impreza.
Performance-wise, it's a slightly different story, as oil in the intercooler will reduce its cooling efficiency to a certain extent. If you're worried about that, the only way to avoid it is to fit a catch can, or if you can't be bothered with that you could remove your intercooler every few thousand miles and flush it out with degreaser.
Plenty of guides and other info on catch cans here if you do a search.
Ollie: That doesn't look like a lot of oil but photos are never the best means of determining just how much there is. Add a breather to the crank case and each cam cover and you will get rid of the problem if it is indeed a problem.
Poor previous rebuild by the previous owner, i saw the signs when i bought it (reciept wise) that something wasnt right, but heart before mind as the car and engine bay was immaculate.
All good now though after Scoobyclinic did their thing with it