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).
Grumpy ole man with over 50 years towing experience. 2012 F-150 Lariat SuperCrew EcoBoost, 6.5' bed, Platinum White/tan, Leer 100XQ camper shell, Nomad Joey 196S TT, CarMate 7x14' enclosed cargo trailer, three other utility trailers, plus a retired 2000 Keystone Sprinter 25RKS 5er we rarely tow now. ProPride hitch on the TT. Reese Strait-Line hitch on the cargo trailer.