The other guys are certainly correct in suggesting that the airflow meter is the first thing to check and rule in/out. As Midnight suggests, unfortunately most mass airflow sensor failures do
not cause the check engine light to illuminate.
There are a couple of things you could check physically. The first would be to pop the bonnet and make sure that all the jubilee clips connecting the MAF tube to the flexi pipe, the flexi pipe to the intake tube, and the intake tube to the turbo are all tight. Also check (as far as you can) that there's no physical damage to any of the above. The intake tube (the one running under the inlet manifold to the turbo) can cause these sorts of symptoms if it splits, although its location makes it the most difficult to check.
Also make sure that the carbon canister purge pipe hasn't become disconnected from the flexi tube.
While you're there, examine the Mass Airflow Sensor itself and check for a stripe of green paint (and white text) on its mounting flange, like this:
If you don't see that paint/text, your sensor is a good six+ years old and is a candidate for immediate elective replacement.
Finally, if the engine is obviously misbehaving at traffic jam/parking speeds, one way you can (indirectly) test the health of the airflow sensor is as follows: Stop the engine, disconnect the sensor (i.e. unplug the grey connector from the sensor under the bonnet), and restart the engine. You will see the Check Engine light on - that is to be expected. Drive around at parking/traffic jam speeds and see if the engine misbehaves noticeably less. If it is more consistent, then you have likely found the culprit. Stop the engine, reconnect the sensor and
stay off boost completely until you have the new one fitted.
If that test doesn't demonstrate an obvious difference in behaviour, and if your sensor does have the green stripe, it's up to you whether you want to take your car to a dealer/specialist and have it diagnosed, or whether you buy a new sensor speculatively and see if it improves matters.
If you decide you do need a new one, the part number is 22794AA010, just over £100 from a Subaru main dealer, just less than a ton from somewhere like Import Car Parts or AS Performance. If you do fit a new one, reset your ECU immediately afterwards.
Oh - and if anyone pops up on this thread later telling you about a "test" that involves disconnecting the sensor with the engine running and seeing whether or not it stalls, ignore them. That "test" is meaningless and proves nothing, one way or t'other.