Generally a lurker here (just interested in proas) but I am an experienced sailor. Symmetrical sectioned rudders and foils in general generate lift simply by angle of attack (what you feel if you hold your hand flat out the window of a speeding car and then tilt it a little). Drag increases but lift increases more up to a given angle of attack, so it is beneficial.
It took me years to understand that what I was taught was sideslip when sailing, and was taught it was bad, was actually a positive angle of attack by my keel/centerboard/hull and was good. Same for a small amount of weather helm. The hull itself will also have a positive lift/drag ratio at small angles of attack, so generally when you are sailing you will want a little "sideslip" and weather helm on. The degrees talked about are usually in the 2-4 degree range. Usually the optimal angle of attack for the boat will be a compromise between the best lift/drag angles of the hull and each foil.
There has been a certain amount of experimentation with gybing boards and trim tabs on both keel/daggerboards and rudders- all attempts to create asymmetrical foils that will work on both tacks. Many modern racing monohulls that use a canting keel for righting moment use twin asymmetrical daggerboards for lift. Asymmetric daggerboards in the floats of multihulls are also common.
There is no question that one benefit of the proa concept is that it could easily have asymmetrical foils, assuming they pivot around the leading edge when shunting. Shapes would have to be compromised for turning ability, but are much more easily built and used on boats that are asymmetric side to side than boats that are asymmetric bow to stern.
Jesse Deupree
F-31 SORN
Portland Maine