Afzal, I use 'normal' (eg Hoya) filters including coloured ones for B&W, and haven't noticed any vignetting other than the light fall off that the centre is for?
Hello Ben...I presume you are another Aussie!
Is there a reason to keep a UV on when the centre filter is on? Apart from protecting the more expensive filter, of course, but this will be at some small optical cost.
I suspect the major 'problem' with the polariser is assessing its effect rather than vignetting...anyway, such things are not a reason not to buy the camera.