99% of the buying public is not going to bolt a turbo to their Outback. Even with a kit from a tuner, it's still a huge PITA for most people, and you will void your warranty. The reason these cars come with small turbos is threefold. Marketing (can't have the grocery-getters outstripping the STi's), reliability, and drivability. Most people could give two ****s how much power their car makes at 5500RPM, they want it at 2000 or before. The 2.5 boxer is a big step up for Subaru being able to market turbo engines to the masses, as with the 2.0 they'd taken a different approach than VW or Volvo had to the market...big turbos, long spool, everyone except teenage fanboi dynopenis no likey.most factory turbos are wtfsmall anyways, I say if you want to turbo it for anything above like 1psi then your best bet is to get a n/a motor and boost it yourself. Granted, Subaru has vehicles geared for performance so their turbos will probably be larger than most other factory turbos, but you can still get better performance on the N/A model by boosting it yourself.
Then you get the option of turbo size (which affects boost pressure and turbo lag to a degree, larger turbo means more centripetal inertia which means more turbo lag, but you can get higher volumes of air through it once it's spun up) and psi.
/durnked.