just thinking, to go to 12:1 cunno theres something wrong with the set up of the rig. The cunno should be making the mast bend forward to stretch the sail. this won't happen with inline spreaders.
I use raked back spreaders 15mm forward of the mast track and with "27" on the loose gauge has already "prebend" in gthe mast. The sail has a luff curve and when gets windy the you need to stretch the curve out with cunno and outhaul.
To explain it well come to the states or nationals or any event where there is a mob of us and i will show you how stuff works, and the latest gismos as i have ready made parts for mossie cobra and arrow.

I nearly go to every event in vic and exploring interstate (wildcat regatta, Great lakes NSW)
I just gave Brett and dave from QLD some lessons on rig set up.

There is also a tuning guide on the Mossie website.

Matt