Robin2:
You are correctly describing how you (and other sailors) use tell tales. But since you can't see the wind direction upwind of the sail you don't see just how much the sail is affecting it. You need to look at wind tunnel pictures using smoke plumes to show wind direction. Think about it this way - if the sail didn't affect the wind how would it generate any force?Of course I can quite see how you might wish to pursue it anyway just to see what happens. My original concern was that you were focused on issues of PID fine tuning without thinking about the external "mechanical" issues.
I don't see how the telltales vs sail, effect things for the autopilot. As long as the wind vane (PIDInput) is a bit in front of the jib (and the rest of the boat), it should not be affected by the boat itself. It only senses apparent wind angle, which is the only information needed to steer to the wind. The telltales is just a means of displaying this aparent wind angle to the human eye. Sure they are affected by the sail in all sort of ways, but I don't see how that effects the working of an autopilot. Or am I missing something? Are we talking about different things?