ok, seeing as we've had them for the past several years (albeit with fridge units stuck on top and the high top versions) i can say for sure that the more powerful ones do give better ecomony, but only just

our newest ones (110 brake) had trip computers and, driving like miss daisy, the best i managed was 46 mpg on a day of mixed driving, town and country
the same sort of mileage, but foot to the floor, saw this drop to 31 mpg

on our 90 brake versions (from 54 all the way to 59 plates) would get between 26 and 34 best to worst

we've had them without the fridge units too, as hires, and they were between 20 and 25% better of fuel for the 90 brake ones so, the best i would expect them to do is knock on the door of 60 mpg if you were doing 56 on the motorway

around town, in london and driving like a londoner i've had mine as low as 24 mpg so they really do depend on what your right foot is like

they also have a titchy fuel tank which may also explain their limited range, some of ours are much thirstier than others too, identical vans doing identical runs and one will do around 280 miles before the light comes on and the other will still have 1/4 of a tank left showing