When you say Auto do you mean the CVT? The CVT can't really hold much more power than stock.
Otherwise as much as it might sounds stupid. When it snows find a big area to play in. I do it to have fun but to also find the slipping point, gripping point, how to properly correct the car and where the suspension needs helps. Snow and in dirt are the best to really test the car.
A lot of people feel invincible with there Subie while others still drive like a grandma.
Also make sure to really be good about your oil changes. I don't care what Subaru says... you have a new engine. Oil change @ 500 / 1500 / 3000 an then every 3k from there.
05 WRX Wagon - 409 AWHP / 334 WTQ Wicked Matt Tuned