Gill,

I think Marcus said what can be said on what to look for. I am no professional sailmaker or racer, but I have my theories and experiences. Based on that, I think a spi needs a long luff and an open leech.
If there is too much draft in it, it will have too much drag and not be fast. It will feel powerful, and it really will be powerful, but not fast. Power is not always fast..
A long luff generates more power for not much more drag (Klaus, rein me in if I am wrong). If the luff is designed overlong for the distance from pole to mast, the sail will be hard to trim right and you can not generate enough apparant wind with it and will have to go deeper than optimal. The pet peeve I referred to earlier is that I think current spis are designed and set with a luff not tight enough. The Tornado is getting there, but I still think it should be a bit tighter.
An open leech will not backwind your mainsail but help airflow over it. Twist is designed into the sail to help the leech open.

Measurement rules and what shapes you actually can make and fly while sailing are all constraints you will have to work within. If you want to study shapes a bit, look closely on the luff tape from different angles on a 49er, Tornado and Vx40. Try to find pictures from 90deg to the side and from straight ahead.