What kind of boats are you losing places to? Other F-16s of the same make with the same sails?
Are the boats set up alike?
Are you sailing the same angles and is your crew weight similar?
How does the skill level at downwind sailing on cats with spis compare?

There is a lot of skill involved in downwind sailing under spi, especially in the lower windrange.
Your spi remind me a lot of Tornado spis, I guess Landenberger is transfering technology. Funny shape at the tack, almost looks like it have stretched? I dont agree with John that the luff looks a bit tight (I like it), but is the spi set up according to Landenbergers recommendations?