You can't say it burns the same amount per sector unless all the other variables are absolutely identical. CI0 versus M79 may only save 40kgs. But it counts. You can't measure that saving even if you assured all variables where the same. Fuel is becoming more and more valuable than time so, yes, you may save 10 minutes air frame time at a higher speed but if the airline gives you a low cost index, it means it, even if it's M76. Boeing made up the aircraft AND the perf figures!
Drive your car at 50 versus 70 next time and see the benefit. But I can't for the life of me think why!? I save 20 minutes at 70!?