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Wind Foiler said..My experience (having spent the best part of 6 months on the Alu-Foil) is the stabilizer doesn't create extra lift, it creates extra stabilitly. I know that sounds wrong but i havent found any of my foils to create extra lift by adjusting the stabilizer. I havent found a need to change my mast track position when triming my stabilizer wing. It does trim the angle of the board, and I think the efficency of the front wing (like flaps on an aeroplane, creating lift at lower speed but there is a trade off, ALWAYS). So for beginners.......2 washers. Slower speed flight (like a plane landing)

There are a lot of semantics that could be argued about this topic but ultimately it does increase the 'lift' of the foil as a system because it increases the angle of attack of the front wing, which as you noted can be seen by a change in the trim angle of the board...and which you kind of explain in a round about way. As you also noted it increases stability because it increases the longitudinal dihedral of the two wings (also referred to as decalage). This is an important point because looking at aircraft, a monoplane which has zero or very little positive decalage is very unstable in pitch. This is what is happening when you trim the rear stab neutral (in NP terms), you reduce the decalage and hence stability....but as you note, you pay a penalty for this with drag. Having sailed an Np ally foil with the neutral NP trim (IE no washer), it is very unstable in pitch....compared to say an F4 setup for racing for a starboard race foil.
Personally, having tested every possible wing angle (on the F4 flight) you want to run max stabiliser trim (or max decalage), particularly if you are racing, it is the fastest and also most comfortable way around the course. Yes you pay a drag penalty for it but the added lift through front wing AoA and the fact you can run more power through your rig comfortably because of that makes it faster.
The only possible reason for running neutral of negative trim on the rear wing would be to make the foil feel more like normal windsurfing (more back foot weighted) or add a perceived 'safety factor' which I think is not the right way to go about it. In my opinion