A beachcat mainsail have about 10% draft when designed and built.
A rotating mast adds from 2-4% depending on mast.
Pulling on the downhaul will in my experience bend the mast and effectively flatten the sail.
After flattening with downhaul the leech can be sheeted in some more for additionally improved vmg.

A loose leech is the monohull way to reduce draft in low wind conditions ;-)

Try the different proposals and see what works for you.