Out of curiosity, after filling up a couple of days ago, I put the car into ECO mode before a 30 mile cross country drive. Fairly flat, B-roads/country roads, light traffic, within the speed limit, but leisurely pace.
The car definitely felt a lot more sluggish (relaxing?), seemed to rely on the hybrid system a lot more, and sounded a lot quieter too. Amazingly, the readout showed 36.2MPG when I got to my destination.
Not sure I would do it again though, too much of a compromise. As I said, economy is not my priority and if it was I would have bought a different car. I'd find it a shame to spend my money on a RX, and then not enjoy its performance!