David,
first of all my sincere thanks for the vane design. It has made my sailing so much more relaxing, that words fail me!
I have experienced with one and two part sheet, and haven't made up my mind about which to stick to. I dislike the amount of rope with two, but with one I seem to have more problems with over-tight upper part of the sail than getting too little twist - and I haven't even installed a Arne-type anti-twist sheet.
As said earlier, the vane works very well. However, I have also wondered about if the vane oar has too much character of it's own. And I have the problem of fouling to the prop, which has even led to damage and re-glassing of the oar once. I have to tie the oar whenever I use the motor (I have a setup which allows me to lift the wind vane system up, although not fully).
But lee helm? This is something I have not noticed, not with the vane or when hand-steering. If something, I still have quite a lot of aggressive, "dinghy-like" weather helm whenever the sail catches a gust.