This is a friends car and I have not seen it. I become concerned though when she said the ac had to go off to get up hills. All the obvious stuff has been changed, air filter, plugs, pcv and other regular maintence items.
It may just be the 2.2 as she is used to driving a Blazer
and only drives the Legacy once in awhile when the Blazer tows a boat.
I am thinking something is plugged up. The car has all highway miles on it. Maybe the cat is plugged? Maybe a motor vac would help? and I really do not know what to look for other than the stuff ^^^ up ^^^ there.
It just sounds to me as though more damage is being done so any ideas at all would be helpful. There are no warning lights on at all.
I had an 83 Chevy van that was a real dog when the flapper valve shut off the exhaust and got stuck. A part of the Emission control system to increase back pressure to the EGR. I don't know if the 2.2 has anything like that.