Hi mate, glad to here that you are enjoying the new forester.
Just a thought - but the burning clutch smell might be the hill holder. Are you are trying to reverse the trailer down hill? If you hit the brake pedal while the front of the car is pointing uphill and you are in a gear (even reverse) the hill holder will kick in and the brakes wont release.
I have Seafeaer Viking 5.5, at the moment I tow it with my 2.8d hilux, I don't go anywhere in a hurry but it pulls it up the ramp in low range no problem... The wife drives around in 06 XT so Im considering trying it out with that after yours and others encouraging words about the XT. Not sure what its weight is but it would be close to the same as your boat. What out board do you have on the back? 2 stroke / 4 stroke?
Cheers
Mat.