I think listening to your sailmaker is excellent advice. If one is able to fault or better ones sailmaker, perhaps it is time to switch sailmakers or become a sailmaker <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Sailing to our telltales is what we have always done. Sailing without telltales but to the luff curl is not so good in my experience. We set up our spi the same way in all conditions, but how hard we sheeted it differed a lot depending on how fast we went and what angles we aimed for. The latter is part of why some teams are blindingly fast while those who sails as two individuals on the same boat are slower in my opinion.
Not much hard facts there I am afraid. One piece we were always looking for was the switchover from "low and slow" to "fast and high". We found that in very low winds it paid off to not try and build so much virtual wind but sheet the spi loosely and go deep. At a certain point, we could sheet in a bit and build enough virtual wind to get better VMG.

Simon,
I would say that it is easier to make a spi with a flat cut deep by trimming it than making a spi cut deep flat by trimming it. Agree?