The bigger the cushion, the better. It's recommended to not come that close to your max rationg on a regular basis. The vehicle will have a GCWR rating as well, which is the weight of the everything (vehicle, trailer, all the stuff in said vehicle) you can use to compare against. While I'm not the biggest fan of a unibody tow vehicle, it should move it alright. My concerns would be with how well the vehicle would be able to stop with a trailer, and how much the trailer will control the vehicle when towing. I do not know enough about the Subaru Ascent to give an opinion on it. It may be just fine.
For reference, I pull my stratos 200xl with a diesel canyon. My tow rating is 7,600# with a GCWR of 12,700#. I think my trailer weight is in the realm of 1000# heavier than yours. I have plenty of power pulling the thing and can stop well, but I can definitely feel the boat back there.