Foilstate -- thank you for that excellent breakdown. I'm on F-ONE but find I'm going straight on anything overhead with 790HA or 840, and doing only slow gentle turns on head high. I knew that the smaller the wave was, the more I could turn but you have refined that observation in a much more meaningful and helpful way.
I was on Maui for 3 weeks, and the 940s. I was surfing like a shortboard on anything my balls was allowing me to do. I'm not saying I was gracious and rad, but I had lines I would have with a shorboard. So about 8 ft I was fully confident...on bigger than that it was more my balls then the foil and I was trim and glide and escaping the pitching session at all cost, but that was the rider not the foil. I'm surprise the 840 was locked in. I could understand the 790 because of the 86cm wing span, the 940 is 78cm wide.I was using the C195 Surf stab, I don't know if it make a difference. I was on the XX-Short fuse, but I would have prefer the X-short I think for more stability at high speed, probably not used to surf at that speed.
One thing to consider is your stance and body position on the board. Make sure you are keeping your weight centered and balanced over the board, and that you are looking ahead and not down at the water.Another thing to consider is your riding technique, specifically your edging and steering. Make sure you are using your back foot to edge into the wave and steer the board, rather than just leaning or shifting your weight.Finally, make sure you are riding the right size board and sail for the conditions. The 111 litre board and 4.2m sail you mentioned may be too small for the wind and wave conditions you are encountering, which could be contributing to your difficulty in control and stability.
Just adding being back from a 7 day trip to the Canary Islands (Fuerteventura) where we winged 7 days at least 4h a day in small to scarry high waves:
Get to a proper surfstance and bend your knees !
I,m notiours for standing up too straight, but the minute it got exciting and I started properly bending my knees and getting in lower attack position I would use with prone and sup surfing everything got better: more control, the ability to use more speed and to tighten up my turns. I,m 95+ kg and only brought my Cab H-series 1200+1000, but if I had brought my 800 that would be even for my weight the one for the bigger waves. Not having that one with me, it is suprissing how much you can get out of a 1000 by using the proper active stance.