The optimal speed for the Model S (to maximize cross-country speed, taking charging into account) is actually closer to 90mph.
At 90mph, the range is about 200mi, and the optimal strategy is to use only the lower half of the battery, driving it from (say) 50% down to 5%. So you'd stop to supercharge every 90 miles. The timeline would be something like this:
Drive 90 miles @90mph (1 hr)
Recharge from 5% to 50% (18min on fastest supercharger, + 2 min overhead to decelerate/plug/unplug/accelerate)
Repeat.
This gives you 90 miles every 80 minutes, for an average cross-country speed of 67.5mph. Doing this continuously for a year, you could log just shy of 600,000 miles.
I took delivery of my Model S P85 (VIN 00061) on Sept 24, 2012. If my battery pack had been enabled for 120kW supercharging, and I had followed the above schedule, I'd be at 1,375,000 miles by now