There are many different cables on the market for VGA and some are better than others. The ones I stock are very high quality PC monitor leads and the ones sold by the Media Factory are even better still.
What you will find is the the lesser ones are not constructed to the same high stanard. They might only have basic screening or none at all. Ultimately, this does affect the picture quality.
I do encourage people to try different cables, but if you're looking to hook up a £3.5k+ plasma, please don't cut corners! You've bought the best, use the best to get the best.
All the best,
Dr John Sim.