I know a decent HDMI cable will make a fair bit of difference when compared side by side, picture for picture with a cheap crappy HDMI cable
How sure of that are you? What kind of differences are you talking about? Are you sure you're not comparing composite to HDMI? As two different HDMI cables will look the same, there won't be any difference unless there is interference, and then the image on the TV will be broken up/corrupted etc.