I used your advice even more and set up mast foot all the way back -1 cm (FT/9.8) . The board felt really light (conditions were very underpowered though) and was easier to get on plane while a little more nervous in shlog mode (but that doesn't matter). I'll see how that works in higher winds.
The regatta course was typical "8" slalom course (2 buoys on a beam reach), about 50 people, very underpowered conditions, with shifting wind direction.
I finished all the 3 races and was classified as 36th
For me the greatest problem was to get the clean wind after start and due to the weak, gusty wind I was often afraid to "invest" some upwind ground to go downwind and pump trying to get on plane - if I were doing that too much I would have to tack to get upwind from the buoy. The result was that I was planing only in solid gusts when everybody else was planing on a standard gear, too, but I'm thinking that probably I could get on plane more often using my gear (if I had cleaner wind and could pump better).
I find that even with the long fin and various techniques, FT is less efficient in shlogging upwind/on a beam reach than a longer, standard freeride board with smaller fin.
Anyway it was a great experience (my third regatta). The only problem over here is that my FT is often not allowed as being too wide (80cm max, they made an exception for me this year) and in typical forumula regatta I probably don't stand a chance like you said (and this kind of regatta is less popular, too).
Ideally I would like to have a board which can be used with my 9.8, get on plane as early as FT, go upwind almost as good and be not more than 80cm wide.
As I don't plan to get a new board anytime soon I'll probably try to get better on my C-111 and use this board in regattas.