70 mph in 6th is 2600rpm for my manual, stock tyre sizes. By way of extrapolation, I reckon I can hit 155 in 5th just before redline. The 6th gear ratio (0.85:1) is not really any different than the top gear in a 5 speed box, and 1st is only marginally lower. Thus the total gear range is no different than a 5 speed, rather each gear is closer to the adjacent ones. There's less than 1000rpm difference between every gear except 1-2.
I believe the concept of the 6 speed is you're more likely to find the right gear for the conditions, not to provide some super overdrive for efficient motorway cruising.