PTP,

we are discussing design, and I just stated what properties I would look for in a leech, not what a sailmaker would deliver today. Get a really old spi, designed to go slow and deep and try sheeting it in hard. I bet you would see a closed slot and backwinding main, unless you get separation first. You can also get a closed off leech if your luff isn't set up right, in my experience.
I have had the pleasure of owning a spi which had a leech closing the slot and backwinding the main if we tried bringing the virtual wind too high, hence my comment on the leech.

We dont set the main traveler centered, but let it out 15-30cm, depending on windstrength. We also sheet the main quite loose but not too loose, which is slow. We trim the main to the leech telltales until crew is on trapeze downwind, first then we need to sheet the main quite hard (but again, not too hard, which is slow <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" /> ).