OK, now I have a little more info.
Do your use an adjustable outhaul with your big sails?
That could be a real key to extending the range.
If you get to about 14-15 knots, more downhaul and an outhaul adjustment
will get you back to being alot more comfortable.
I think if you use the stock 70 cm fin you will be much faster and be able to get upwind much higher/faster.
You will need to "rail" your formula board slightly to leeward, lift a bit with your front foot in the footstrap and push across the rear of the board in a nearly horizontal (level with the surface of the water) direction and you will find youself flying upwind "on the fin".
I do not think this was possible on the Mongo as much of the lift was up near the board and what you need is lift all the way down to the tip of the fin.
Hope this helps,