Rule of thumb: 20 C decrease in hot section temp yields double the parts life. That is the driving logic for flex thrust (or alternate rating, or any other means of turning down the wick).
And even if the SFC is marginally better at reduced thrust (*), the lower aircraft performance means you're at the lower thrust & altitude for a longer time period. Thus, higher trip fuel burn.
(*) Whether or not reduced thrust improves takeoff SFC is a function of the individual engine model. Your mileage may vary, but usually there's a slight "hook" in the curve.