You can't use it with HSS, the Quadra isn't an IGBT light so can't recycle (anywhere near) fast enough for HSS.
If you meant Tail End Sync then you can't do that either if, as I believe, you have the faster flash heads with your kit.
ND filters are very worthwhile in bright sunlight, but as the Quadra (and especially when fitted with the 'A' heads isn't exactly overpowered, it isn't something that you'll need to use all that often. Personally I use a much more powerful system and frequently use 3 or 4 stop ND filters to cut down the effect of the ambient and to allow wider apertures to be used.
Edit: I used a ND filter on
this shoot