A while back I upgraded from the D7000, 10-24 and Lee 100mm filter system to the D800E, 14-24 and the Lee SW150 filter system. While the camera and lens were big upgrades, the SW150 filter system was a step backwards from the standard Lee system in a few areas.
The large openings at the sides of the holder allow stray light to enter behind the filter holder, causing reflections between the back of the filter and the lens element. The problem becomes more evident as you add more filters.
Because of this reflection problem Lee advise only stacking two filters on the SW150 system, I was stacking 3 filters on the 100mm system with no problems. The strongest Lee 150mm ND filter is 3 stops, I was using 4 stop filters on the 100mm system.
I do a lot of seascape images with shutter speeds around 60 seconds and over, 6 stops of ND just wasn't going to cut it so I started experimenting with a new baffle that blocked out all light except that which was coming through the filters.
Lee do not make a Big Stopper or a CPL filter for the SW150, but there are some alternatives from other manuafacturers.
I have a Hitech 150mm ProStop IRND 10 stop filter, this is very similar to a Lee Big Stopper but is made from CR39 resin instead of glass. I've only used it once but the results were good, it has less of a blue cast than my Big Stopper.
I also have a 150mm CPL filter from Cavision, this filter is 4mm thick (Lee filters are 2mm thick) so it requires 4mm guide rails. The filter is square, it is rotated by turning the filter holder, which means it is not compatible with graduated ND filters as the graduation line will also rotate as the holder is turned.
I only use the CPL for waterfall shots, so I normally have 3 sets of 2mm guide rails mounted on the holder, when I'm shooting a waterfall I'll change it over to the 4mm rails + 2 sets of 2mm rails.