Yagon,

I cannot answer your questions with any authority I am afraid but have some fifth hand info on some aspects.

It appears you are looking to design a board similar to the Exocet Open 310 that was trialled at the Olympic Board Selection in 2004 http://www.boardseekermag.com/special_features/olymic_board_2/olympic_board2.htm

The new CarbonArt Formula Board has no cutouts and some of the locals here have sailed it. If I remember correctly it was reporte as being a lot stickier and there may have been some comments about it being less user friendly in the chop.

I believe the width of the board is more important for controlling the pressure in the fin rather then direct correlation to upwind and downwind angles. An underfinned board will not be able to achieve the same angles. In Formula Racing the mantra is to rig for the lulls, having a narrow board will be severely handicapping in variable conditions.
Someone more knowledgeable may have better insight into the feasibility of using a narrower planform in windy conditions.

I understand a smaller fin works best when it is windy because you are not looking for more fin pressure, its reduced area contributes has less drag and leverage.
Using a smaller fin is a fairly normal way of setting up for higher winds, but most racers will change down a sail size before changing to a smaller fin.

Chris from VMGBlades is probably the best person to comment on your questions. He has sailed the CarbonArt, and knows a bit about fins too.

J