TERRIBLE idea with Subaru AWD.
You want all the wheels the same size, and same sized tire. Routine tire rotation to keep the wear even on the "front tires". If your tires wear unevenly, and you need to replace 1 or 2, you might end up needing to replace all 4 at the same time depending on how much they are worn.
Definitely a bad idea. At my old shop, it was required to void warranties on AWD subies that ran staggered. Its a hefty cost to replace the diffs when the get destroyed from staggered. Seen it a lot on WRX and STi's, a few times on Crosstreks. If you don't care about warranty and REALLY really want to run staggered, look into customizing for RWD or something. Not sure what the benefits would be. There are way better platforms for staggering, Subarus are not one of them.