I wondered whether everyone would just accept my statement that flatter is faster - or whether I would get some arguments!
The first answer is practical. Fast, modern racing sailboats have been using flatter and flatter sails. I have used flatter and flatter mainsails myself in recent years and feel that I have become faster and faster. I was led to this progression by Jochen Schumann (whose mainsails I used to watch from the position of umpire) and Sergei Pichugin who was an early devoteee of flat sails trimmed tightly.
The second answer is theoretic and has to do with drive force and angles of incidence.
Angle of Incidence - is the angle aimed by the chord of the mainsail - the line from luff to leech - relative to the direction of the apparent wind. The less the Angle of Incidence the higher the boat points, but the height achieved also depends on leeway - the amount the boat slips sideways in response to the side-force.
The relationship between the chord of the mainsail and the apparent wind depends on 1) the angle the boat is aimed - limited by the luffing of the leading edge of the jib, 2) the angle of the boom to the centerline of the boat and 3) the closure of the leech of the mainsail - which determines the chord of the mainsail above the boom.
Height - Optimal height is achieved when speed is retained at a low angle of incidence by aiming the boat high, trimming the mainsail leech close and making the least possible leeway.
A full sail is able to achieve these effects: it is a sail with a tight leech, a leech that is close to the centerline when the sail is set properly - resulting in the sail being `automatically` set at a low angle of incidence. It produces a very high total aerodynamic force at its optimal angle of incidence to windward - 17` and still produces an adequate drive force and speed at a low angle of incidence (at which a flat sail would produce less drive force and less speed).
A flat sail is a sail with a flat, open leech, a sail that has a small camber. Its surface does not deviate much from its chord - the line from luff to leech.
Speed - A flat sail develops less total aerodynamic force than a full sail but a greater percentage of the force is directed forward (as Drive Force) and less laterally as side force (leeway) and aft as drag. Consequently a flat sail produces more speed than a full sail which dissipates its greater total force in side force (heeling and leeway) and drag.
Sailing one degree closer to the wind (when the distance from start to layline is one mile) results in a reduction of distance to the mark of 130 feet, so one would think that the height achieved by a full sail would more than compensate for its slowness and that full sails would be faster overall.
However, a flat sail with its leeches trimmed hard to reduce the drag angle and the angle of incidence can sail as high (at as low an angle of incidence) as a full sail and seems to retain its speed better.
My commentary had to do with correcting deficiencies regardless of the sail being used. Every sail has its optimal angle of incidence and optimal drag angle so as to produce the greatest possible speed made good (VmG). However, in moderate air from any given setting, if you want more height, trim in - if you want more speed, ease out - in small increments.

Dear Prof. Walker, thank you very much, for your open-minded trimming advices. It is fascinating to me, how analytically you break down the problems to the basic adjusters. Most of us, I guess, work through the huge parameter space much slower with trial and error :-) Unfortunately, I have not yet captured a basic advice. It seems counterintuitive to me, that in light to moderate air "fuller sails are higher but slower". Can you explain the physical reason to me? Thank you for your help, Tobias Hanke

Thanks Stu, once again a fantastic contribution to the Soling class, and also general sailing skills... cheers matias