i use the p system - the wide angle filter holder (one slot for filters only) with my UWA.. but i don't use cokin filters, i find them a little too overpriced for the quality (yes), even compared to singh-ray. might as well get tianya - cheaper, frankly more neutral.. and not too shabby in terms of image degradation.
not many people use gnd filters in singapore - in short either they feel the desire to use it, thinking (sometimes wrongly) that they can recover the details; or some will profess that you can always do it digitally.. which is not off the mark if you merge a few frames, but it is a hassle, much more than just using the filter while shooting.
entropy - there is slight vignetting at 10mm, just darkening of the corners if you use the wide angle filter holder; please do not get the 3 filter holder, you can see the holder in your viewfinder. just stop down, from f/16 you will see much less vignetting already.