yes there is a difference in mpg but it's really only any good in town driving, the main thing is it's in a cheap road tax bracket compared to earlier fl2's and this is the reason land rover did it. stop/start is standard on all 2010 models onwards
You need to bear in mind though, there is alot of stuff that will cause the system to NOT work and this is not a fault with the vehicle. Things include.... cabin air temp not reached target temperature, not wearing seatbelt, you haven't made a gearchange, terrain response turned on, towing a trailer/caravan, battery voltage low to name a few!
There is a good thing built in though, restart after stalling. Bloody good this is. If you stall it, dip the clutch and it restarts, marvelous!