I was just helping a friend with a Forester look into options. I had some trouble looking up HIS exact vehicle. It seems there is a bit of variation depending on model, etc. so do a VIN check or whatever you need to to be sure you are researching accurately. His payload was impressive @ 900# (only 200# less than another friend at the campground with a Ram 1500 but that's a whole other story :))
The pulling power of the Subaru is by far the limiting factor. I think we figured he was good up to about 2000# total in the vehicle and trailer so he was looking for roughly 1500# loaded tear-drops and popups. There are some good options at that weight.