Overall, I'm finding highway driving to be sort of draining on my little I2. I went from 49.7 mpg the first 80 miles of this fill-up down to 46.5 mpg after about 40 miles of highway driving - if it's anything other than flat, the tiny ICE really has to push itself to keep 70mph.
I havent noticed any significant difference in ECON on or off when driving interstate. Other than on having less ondemand power to get around someone quick (i.e. someone even slower than me in right lane and speed racers in the left lane).
The big diff to me in terms of ECON is in town. I run with ECON off for the first few miles after leaving work because the heat is unbearable. I cant deal with the sweltering heat during the first few long lights at the start of my commute home.
I have driven I35 from Dallas to San Antonio quite a bit over the years (not in a hybrid though). The low rolling terrain (there are ever-so subtle hills) might be hard on the battery pack... like running out of steam on the uphill because the downhill didn't give a chance for the regen to charge enough. I get into a similar situation around central Arkansas... short choppy hills without a steep enough down slope to coast at speed before burning battery out on the hill.