John,
Yeah I thought you had a loose foot to keep the power up, 170kg needs power !
Looking at your mast rotator position leads me to one of two possible conclusions based on your comments :
-If you eased off on your diamond wires to get less pre-bend, your mast would bend in it`s min. axis, so you`d get an even bend in the mast when rotated as you have, leech would open fairly evenly.
-If you raked spreaders forward to get less pre-bend, but have tight diamonds, your leech would only twist off above the hounds, where the mast can bend in minor axis.
So in effect you would be getting the top of the mast to bend to weather in it`s minor axis, but with lots of rotation allowing that to bend toward the leech, letting the top twist off ? (I`m assuming you eased off on the diamond wires only without changing the spreader rake, or you wouldn`t be worried about the mast sagging into the slot when derotating.)
With this much rotation you are not letting the top of the mast "fall away" to leeward as with that much weight on board you probably don`t need to.
Am I on the right track here ? Just a thought - with that much rotation are you not getting a disturbed airflow over the luff of the main ?

Steve