Didn't think an air filter made any real difference bar noise and not having to replace it? When i had the map done Andy said not to bother as the standard item is more than adequate. Note Prodrive don't alter the filters either, apparently standard airbox is good for 350bhp!
So benefits:
More noise
Works out cheaper over 40,000 miles
Downsides:
Doesn't filter as well (more air also = more bits)
Initial cost
that's about it!
If you just stick with the standard one SIDC discount will give you 10% off. I think they are about eight quid from Subaru.
5t.