I have wondered about that also.
I have wondered that if the gap between the jiblet leech and the main panel is too small, either because the built-in jib sheeting angle is too small and/or the camber on the main panel is too big, then the airflow off the jiblet leech will impact on the main panel camber, causing it to collapse and flutter and reduce drive. Simply too much air being forced through too small a gap?
How would that affect the airflow overall?. Would the 2-part sail essentially start acting as a single sail with consequent reduction in performance?
An interesting question, Jami. Hopefully some of the more aerodynamic will have an answer.