I can't say that I have noticed any difference in the gybes with assy. speed fins. I often gybe at the end of the upwind slog when the wind is a bit westerly and I can't make the bank. These gybes turn into the classic 'Alpha' symbol shape with a cross over of tracks. Some have been in the low 20's on the is50 with 20-23cm assy fins. The fins I use always seem normal in the gybes given the speeds are usually a bit lower than a full on Alpha attempt. Those gybes usually go from wrong side to right side. Gybes at the end of the speed course usually have no hope of making the proximity circle after a broad run and then coming back on the wrong side.
Have you guys tried putting out some markers yet to judge your proximity?