Ok, so I put the snow tires on the day before we got our major snow...back in November. I have not tried the OEM Yoko's in the snow. I would imagine that they perform as well as expected for all-weather tires. Nevertheless, I probably would have had some trouble in the deep/loose snow without said snow tires. They are amazing in the deep/loose stuff. Nevertheless, I am sure that the Subie awd system is the major factor here. The tires just give it extra help. If you go in the deep stuff, turn off the traction control.
The bonus with dedicated seasonal tires it that they are not too expensive and they are an investment in safety. Also, they will make your all-weather's last that much longer. I got mine from TireRack.com, steel wheels, tires, hubcaps, etc. all mounted and ready to go. I am sure that you can find some used wheels on craigslist for cheap and then find a bargain on some snow tires. Of note, my '09 has 17" wheels, the steelies are 16". Tirerack recommends taller sidewall tires with more narrow widths for best snow traction. I bet you could find somebody looking to sell some 16" oem Subaru wheels, just search the forum to see which oem wheels will work as they have different offsets. Good luck....
Also, a major improvement in the slushy and sloppy stuff with the snow tires. They do give you an extra bit of confidence, so don't push it too hard as the laws of physics still prevail.