[teoman]

What type of car would be efficient for high speed cruising, say 100mph, 160 km/h.

I know this is not entirely in the spirits of ecomodding/driving but sometimes it is just more economical to go fast. For example if you are many people in the car. Or in my case, if I can go fast and avoid rush hour traffic I can see a significant gain in mpg. Getting stuck in traffic sucks. And mpg goes up. (I once got 43l/100km. Going up hill for 2 hours at 3km/h) I could have walked that distance significantly faster. (I have started to bike to one of my workplaces 20-km one way 40 km round trip)
Sometimes blasting on the highway to go through a densely populated area at traffic free is an fe plus.

So, what would be a good candidate? A vw polo or a Passat variant have the same cdA of 0.66 so in this respect would they both get the same FE?

Aero mods would provide a significant advantage, also a gear swap if possible would make the most obvious advantages in my opinion.