I can't tell any difference when tow/haul is engaged unless towing heavy in hills or mountains. The big difference when towing heavy in hills and mountains is the gear selection strategy. It will downshift much earlier for comin' down the mountain, and automagically downshift all the way down to 2nd gear if necessary to control the speed. Without tow/haul selected it won't downshift nearly as early or as deep into the gear selection.
Is it safe to leave it engaged while driving at highway speeds?
Yes. Whether towing or hauling or not, you can drive at normal highway speeds while tow/haul is turned on, even higher speeds such as 84 MPH ticket limit on I-10 and I-20 in far west Texas. But you don't gain anything by turning on tow/haul if you're not towing heavy, and I hope you're not towing heavy at over 65 MPH. (Most trailer tires are limited to 65 MPH).