I love the GND and ND filters and always carry it out. Even in broad daylight with no water, you can get some clouds movement over 60 secs exposure and the pix looks and feel different.
For the wide angle, I dont use a holder, just my fingers holding the filter against the lens and then "freeze" for 60 secs or whatever duration. Ha.... sounds spastic and I've gotten odd looks before. The wide angle single filter holder allows use from 17mm or so and mine's 11mm at widest and becomes 16.5mm with the crop factor in..... so sad. Just that much a diff.....
I hardly use the Cokin P holder when using filters except using them in making panoramas where i need some decent placement across all frames.
Mebbe about time to get one of the ND1100 or something....can't remember... someone told me before of a Cokin P filter that stops down about 7-8 stops at one go.