I'm still far from convinced that the prop would have "overtaken" you, even at 180kts with 40 to go. I would say that the prop was already ahead and the controller wasn't going to bust a gut to get you in front.
I know that there are many many variables that determine "economical" however 250kts in the majority of cases is more economic than flying 320kts+ trying to nudge in front (which is what would be required). There are also many more variables that you probably aren't even aware of in the cockpit which makes the controller choose their way of control regarding airspace orientation, danger areas or just other traffic.
Nothing is ever as simple as what it may seem.