CanonFanBoy
I use the LEE filter system (100) for landscape photography.
I have stacked big & little stopper as wel as stacking with 0.3 and 0.9 ND filters.
I had no light leak. The big and little stopper come with a rubber border that thightens the filter in the system. The reason for that rubber border is to exclude light coming in - as you probably know. The 0.3, 0.9 and 0.6 do not have these rubber border. Neither do the soft and hard grads (I do own them as well)
Stacking the big and little stopper does give you 16 stops. The vignetting with a wide angle lens is very big, I noticed with the 16-35 f/4 IS
But even if you would experience a light leak, you can solve that by covering the top of the lens and the top and side of the filtersystem. If you have only a few minutes to take that shot in a location you are just once, you may take out any risk of missing that specific shot.
As I said, I don't have that experience and really like working with the LEE filter system.