You wanna autocross a Forester, go for it! But, if you're serious about being competitive in autocross, it's best to search through the results of the previous years nationals to get an idea of which cars are competitive in each of the classes.
Not practicing what I preach, I've been running my Impreza L 1.8 liter pretty much uncontested in H-stock. It's not much fun running alone. If I run an index class, I get my butt kicked because the 1.8L is not competitive in H-stock. H-stock is the bottom class, so there's a wide variation in performance within that class.
Rallycross on the other hand, is a different story. In my first ever Rallycross, I did pretty good against 2.5RS's, Galant VR-4's, and Audi Quattros in PGT!
Subie Gal: The Forester didn't run in PGT, what class was it in?