In my opinion, the important part about the leech is that it should not backwind the mainsail. Getting the balance between a open leech and well setting luff is hard, especially if you want the spi to be easy to trim and fast in different windspeeds. I am looking quite hard into the topic right now (from an amateurs viewpoint), and it's quite hard to get right.
Rolf,
I usually keep my main centered and sheeted relatively hard on downwind running the spin. It seems like you owuld have to REALLY oversheet the spin to cause backwinding of the main. If the wind were a little more crazy and you travelered down the main (which could arguably power you up more I guess which is not what you would want in high wind) then you might get some backwinding but in those conditions you wouldn't be sheeted in really firm on the spin anyway. Just wondering... not sure I have enough experience running a spin to qualify my opinion.
