RE: iSonic 111 trim set-up
Did you compare performance or only feel ? One of the trends of the iS111 is (as you comment ) to ride very flat, which is a positive and should not be confused with "sticky" which relates to board not totally breakingthru into top gear (and is more a performance issue than feel/sensation). One of the "problems" (???!) of iS111 is it feels very calm ("lies flatter") on the water, which can be misinterpreted to mean "slow" compared with a nervous or flighty ride from some other designs. Often the opposite is the case in reality, and the calmer board allows the rider to "push on" harder/further at the time that the nervous board is getting a little too flighty...
Obviously (if you were underpowered on 7.8m ??) then adding the larger fin would be an option if you really wanted to generate more lift (or more correctly the feeling of riding more on the fin), but in general powered 7.8m slalom conditions you should be fine with 36cm.
Cheers ~ Ian