I think a lot of it comes down to driving style rather than speed - good forward anticipation, smooth adjustments of speed by looking at situations ahead and easing off the pedal rather than braking when you see brakelights in front, are the key. This doesn't mean driving slow, far from it.
Also, driving in too high a gear hurts MPG, if you're in a gear such that the engine is off boost and has to overfuel to spin the turbo up again every time you try and increase speed, you'll use more fuel; lower rpm isn't always best.
My remapped 130
Ibiza used to return 48mpg long term (calculated from brim-to-brim tankfuls, not trip computer) and could very easily do low-mid 50s on a single journey, without dawdling. It had such a long 6th gear (37/5mph/1000rpm) that I only ever used it above 75ish.