Losing power at higher elevation is just physics; thinner air, lower compression. I'm not familiar with the '05, but am guessing that it's fuel injected; it [I]should[I] adjust the fuel to the air mass on it's own. A clean air filter helps, too. If your tailpipe is sooty, it's running too rich. If it's carburated, you might have to rejet. Not sure how all that affects an A/T.
I just put Blizzaks on my '10, but plan on taking them off in the (late) spring. I really like them so far, but hear that they're too soft to run all summer. I've ended up running dedicated seasonal tires; I used to run all-seasons, but found that I didn't like the compromise. HTH, and have fun with the snow!
'10 Forester X M/T
Fraser, CO, USA