I totally missed this question, but in case you find this answer:

It can barely keep up at true highway speeds. It can keep up at ~60-70 mph on flat ground, but just barely. Given highway speed limits are 60 mph minimum in most places (meaning traffic goes at 70 mph), this means I usually am losing charge on long journeys. If I lived in a place like TX where the speed limit was higher, then the REx wouldn't be able to keep up at all and there'd be a net loss in charge.

However, I have hold state-of-charge (HSoC) on-demand coded to my car (in the EU, you can enable the REx any time you want. CA regulations prohibited this in the US, but it can be enabled via an ODB-II adapter). If I am going on along journey, I enable the REx with a button at 80%.

I've done 600-700 mi trips without charging the car by filling the gas, as long as I'm conscious to begin looking for a gas station when the gas tank is ~10% (which is about 10 miles, and I've found that within 10 miles is about the max you'd find gas stations apart on a major highway).

By the time I get home, I'm down to 40-50% charge on the car (from 80%, since I can't turn on HSoC/REx at 100%, so the first 20% doesn't count for determining whether the REx can keep up) if I'm paranoid. This also is through the Cascade mountains in WA, so the real world usage holds up even over mountain passes. Going up the pass is a real pain since there's a high net loss of electrical charge, but going back down is basically "free" thanks to regen.

From the specs it appears that it can barely average highway speeds in range extender mode. Which is probably enough that the performance for passing and whatnot is the same as the electric mode.