Pete, you sail with the wind at 90deg apparant when going downwind with the spi? Is that a normal apparant wind for going downwind on a F-16? Have you tried going higher to build more apparant and drive off with the higher speed?
PTP: The 90deg rule is a very loose rule. I believe it originated in the Tornado class with a certain setup there, but the class have since gone to a tighter luff. How loose the luff should be depends absolutely on the design of the spi. The trend is for tighter and tighter luffs on fast boats (like your 18HT). I have never seen a Smyth spi, so I dont know how he does it, but why not experiment a bit?
If you let out on the tack line or halyard, you loose much of the draft in the forward part of the spi and move it back. Leech will open more and likely flutter. You will be able to go higher against the wind with the spi like that, but sideways forces will increase a lot compared to your forward forces.
Why not supply some photos taken from below the spi while it's sheeted in its sailing position, and you will perhaps get some more useful responses. Post them here, and I'll run them trough Accumeasure.
If you really want some useful information about the shape of the spi, try to get some photos of the spi while flying. Just lie down under the spi pole and shoot upwards into the trimmed spi. Easiest to get a good result if done on land With such a photo, it's relatively straight forward to analyze hos much draft there is and where it is.