It's like any cable - it will probably work beyond the actual max in the spec, but there's no guarantee, and it will depend on external influences such as magnetic fields, interference, impulses etc.
T&E for instance isn't specced for video work but you will find you can easily send video down quite a length of it, but it will not stand up well to noise from things switching on and off etc.
think I am just going to put in the 7.5m hmdi lead and pull in a cat6 with it with the intention of the customer to use the hdmi if this fails then atleast the cat6 is there and hdmi over cat5 extender can be ordered
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