Well, to be honest, BNC cables do not really make a whole lot of difference. I have never tried, but I do not think I would be able to tell the difference in display quality on my monitor when using either the 15-pin or BNC connection. The thing is, video cards output using the 15 pin connector and you are basically just then converting to the BNC connectors. It is probably the case that certain hardware that ouputs using BNC would provide a image qualityenhancement. But with standard video cards I don't see how it can make much difference.
But the cables at pccables are only $10, so I would said go ahead and give it a try. I have heard that when running at high resolutions, more of an image quality enhancement can be seen.
Oh and I should add that you will lose the DDC capibilites of your monitor with the BNC cables. But as long as you know the refresh and resolution capabilities of your monitor you should have no problems.