I've used e30 90% of the time for the last 25,000 miles on my 2016 5.3 (non-flex fuel). The other 10% was e10 when I'm not near a station with a blender pump. Truck runs great. Torque app shows timing is significantly advanced running e30 vs. e10.
I would have to run a couple thousand miles on e10 to make a firm conclusions, but I have not seen any loss in mpg. Over the last year e30 has averaged about 40 cents a gallon less than e10 here, or about 16%. If the mileage really is the same on e10 and e30, I'm saving about $500 a year running e30.