Let's get things lined up properly first - does this car have an oil cooler? If so then the oil cooler gasket needs to be replaced first, particularly since you said it's by the oil filter.
Contacting Subaru is worth a try but often unhelpful. Hopefully you're the anecdotal success story but what typically happens is that Subaru will offer you a 'sweetener' as they call it in the biz language - it's going to seem good. They'll offer you $1,000 to $500 credit to replace the headgaskets.
The problem lies with the fact that their original quote is heinous thievery. That's normally about a $1,200 job. An independent shop, preferrably a Subaru independent will do it for $1,000 - $1,500 and will install the gasket you want (more on that later).
*** Headgaskets cost $100 from the dealer - so they're charging you over $2,000 labor for a job they can do in one day. They are making serious bank.
Can I fly to your house and do a better job than Subaru - i'll charge $1,000 labor???
The other problem is that Subaru will be installing the same problematic headgasket that's leaking on EJ25's up until 2010 models. And all the dealers I work with do not resurface the heads, though I'm sure they can if you ask. No point in doing that, if I'm doing the job I want a better product in there and done right. I would install the EJ25 Turbo headgaskets which do not have headgasket issues and are visually appealing in terms of sealing technology. Major subaru rebuilders and others doing a lot of this work are doing exactly that. you can ask the dealer to do that if you want, they may or may not oblige.
Folks will come in after me and say that there's an "updated" headgasket but there's no need to discuss that here - you can easily see that Subaru EJ25's have headgasket issues right up until 2010 via a google search or simply search this forum, so they're updated gaskets are not that great of a solution. The gasket they install may very well not be a near %100 solution if current failure trends are any indicator. I suggest taking notes from folks that have done a lot of EJ25 headgasket work themselves.
External weeping leaks are very tough to sort out with pressure checks.
actually it's not tough with pressure checks, it's impossible. they don't fail that way.
the headgaskets leak externally - that's their failure mode. compression tests, leak down tests, and coolant tests are all pointless.
look visually. ileaking at the headgasket mating surface is **THE ONLY*** symptom, looking elsewhere is a waste of time. first step is to rule out the oil cooler (if your car has one, some EJ25's do, some don't), front crank seal and oil pump. the oil cooler is right next to the head mating surface so it can be easily confused.
if it's leaking at the headgasket then it'll eventually need replaced. the good news is that you can essentially keep adding oil and not worry about it. since they only weep externally it's not a big deal. you could put 50,000 or 100,000 miles on the car without it really leaking significantly.
the fact that you just noticed this and there was no complaint about oil loss, marking up driveways, or smoking while driving suggests the leak is really minute and in no need of dire attention. it's likely capable of 50,000 miles easily with no issues...although 2005+ models typically get worse quicker than earlier 00-04 models - those things you could almost never repair if you could deal with the side effects of the oil leaks - staining the driveway, etc, and keep the coolant/oil full.