Re: you wouldn't use a cat 5 patch lead and expect 10Gb/s performance
"And yet you wouldn't use a cat 5 patch lead and expect 10Gb/s performance?"
What, even though Ethernet's got error detection capability, plus layers above and below which typically add correction and/or retries, with the result that poor signal quality may slow it down a bit but mostly you don't see it. (PoE is a different matter).
HDMI, last time i looked, doesn't even have error detection never mind correction or retries, so if the data is wrongly received it stays that way.
So better make sure you use a decent cable. Or maybe not.