I havent't tried the 52 cm fin with the iSonic 133, I have thought that I might be a little on the large side for me, because the 48 cm fin begins to feel also a little big when properly powered.
I was wondering, is there a practical minimum limit in windspeed (around 8-9 knots?) that modern slalom equipment can be used? I guess for the slalom boards there is a minimum boardspeed that is required to keep the board planing (about 15 knots?). So could it be the case that when the wind is too light, the angle of the actual wind in the sail will turn too forward (like trying to point too high) making it impossible to keep the required 15 knots of boardspeed. And in that case switching to a bigger sail does not help much, becuause the liming factor is the angle of the wind, not the power in the sail? So maybe I have already reached that minimum windspeed with 8.3, and swithing to 10.0 does therefore not help?