Closed Trailer with a Std Cirrus in it is 680kg (54% is easily inside the
85%)The sail effect is a bigger decider for me - the trailer can push a smaller
car around when you are exposed to gusts at speed.

I have towed one of those lightweight Nimbus 3 jobbies in a Pfeiffer
trailer with the 2.5L Subaru Outback. It handled better than towing it
with a slightly older (97) Chevy Tahoe 5.7L , ESPECIALLY in side wind
gusts. I imagine this is due to suspension tuning, or lack thereof on
the Chevy's part.
Currently towing a longer and equally light Cobra AS-H26E trailer
with the same car. The biggest towing problem has been "citations of
excessive cross-country progress" awarded by the Highway Patrol.
Both Paul Bikle and Bob Harris' long standing World altitude records
were set within 50km of home, to put the hill and wind situation in
perspective.
Jim