I had a RX400h for 3 years up until last September when I replaced it with a RX450h. Using the on board trip computer the RX400 returned 28.9 mpg over many thousands of miles and after 8500 miles the RX450 is returning 32.8. I have used standard unleaded for both vehicles, but I keep the Rx450 in eco mode.
A number of my friends and colleagues have also driven both vehicles and the difference in the mpg with different driving styles is very marked, both up and down. On a recent 200 mile mostly motorway trip a colleague got 38 mpg, but there were a lot of 50 mph sections!