In our part of the world... the road and traffic authorities doesn't normally relate the words "sensibly driven" with 80-85mph.... (130-138 kmh)... depending on the open road limit (100kmh or 110kmh) exceeding the posted limit from 20kmh-40+ kmh can set you back anywhere between several hundred dollars to over $1,000 plus numerous points loss....
So, I'll take your word for it!
I believe you are on the money regarding the fuel economy penalty of the K02 tyres... probably closer to 15% ... or $.0.30 /litre at current Aus. prices...