HI - and welcome.
This forum caters to the older '89-'94 models, but your '98 is similar. LegacyGT.com and UltimateSubaru.org site may also have good info.
Most- if not all- Subarus (as do most brands) had CAI from the factory. If you look at where your air comes in, it's from inside the fender liner on the passenger side (US-spec cars). CAI usually just move the powerband UP, so if you have a 5-speed you can rev it higher where the HP/torque may be a bit better. With an Automatic trans - not so much.
You can do the snorkel / snorkus -delete mod, which will add a bit more noise.
Any CAI that terminates under the hood is exposed to engine heat.
This is NOT a CAI:
This is better:
Even better (coming from fender):
Remember that anything that pulls air from the stock location, BUT w/out the snorkus will also have a better chance of sucking in water. Especially if you like to, you know, Offroad the Outback.
CAI is a well-covered topic online, so suggest spending time w/Google, too.
GL,
TD