Steve, I also think a tighter luff is faster. Studying photos of current F-16 and F-18 spis show a somewhat loose luff, much more so than the T. But I think it also depend somewhat on what angles you are going to sail, and how fast. If I was going into the F-18 for competetive sailing, I would buy my spi from Gran Segel in Malmø and set it up with a tight luff.

For the other question Catfan asked. You can not go below 75% and still measure in as a spi under most rules, but going much above 75% give you trouble with the leech and shape at the speeds/angles cats sail. Current trend is higher aspect ratio and more efficient sails with less sailarea and chord.