I have been closely monitoring the fermentations of my belgian ales that have been brewed with Chimay yeast. I have noticed, that after an initial very active phase with blowoff and massive CO2, that typically only consumes about 40% of the fermentables, the krausen goes away and there is just a study stream of bubbles on the side of the fermenter and a tiny "ring" of foam around the beer where the surface meets the fermenter.
From what I understand, this type of "low krausen", fermentation is typcicall of wine or champagne yeast. Do you think it is possible that some trappist yeast is a relative of wine yeast from France possibly? That may explain the high alcohol tolerance and fruity esters.
Just food for thought I suppose....