My foray into HDMI cables suggests that there are good gains to be had in AV, but I suspect there would be more immediate gains to be had from improving the power supplies in your HDMI sources (such as your Sky box) and the cables before improvements in the ethernet network were obvious.

Even before this step, though, I'd predict good all-round gains to be had from decent power conditioning driving the TV and HDMI sources. Borrowing an AudioQuest 1200 would be interesting...

Prolific music listener / Hi-Fi addict!