I don't think it will be a mistake.
I don't think enough Americans really know the Impreza all that well (especially those in the Southern states where 4wd isn't necessary).
Also, in general Saab and Subaru aren't sold in the same dealerships.
It's not like the Cimmaron situation, where Cavaliers were being sold in the same room as the Cimmaron. Also, the Cavalier was (pretty much) the cheapest Chevy at the time. Basing a Caddy off of it was a bad idea. The Subaru is not a cheap car. Although it's the least expensive Subaru, it's still significantly more than Civic, Corolla, the Koreans, etc...
For those that aren't aware of the 9-2x's lineage, it will make a nice addition the the very small Saab Line-up (essentially 2 cars for the longest time (9-3 and 9-5)).
And there's really nothing wrong with Impreza. So for those that do know it is Subaru based, they won't have much to complain about other than "It's not pure Saab, and where the ignition is located". It's not like Saab is rebadging the cheapest thing out there. They are giving an already recognized and well reviewed car a second chance by offering it in two markets.
The front end styling is reason enough for some people to be turned off of the Subaru, and go for the Saab.
I think this is a much smarter move than the 9-7. There are way too many trucks on that platform already.