Same thing here: 850 Watt PSU, Gainward 4080 GS.
The cable included with PSU delivers "only" 300W. However, there are no cables rated 450W available in the aftermarket...
PSU spec sheet says it can deliver up to 450W via 12VHPWR.
Why would Thermaltake send a sub spec cable? I want to make most of my systems in 4k resolution, so Is there any solution to this to unlock the last frames by providing the card with the necessary Watts?