Welcome Abdul. I acquired my IS250 SE-L just 2 months ago. My work commute is 5 miles each way on an A road limited to 30 or 50mph. Every week or so, I do a 40 mile round trip to the supermarket (I live in the sticks !) via motorway and dual carriageway.
I observe the speed limits and am returning an average of 29mpg, so your wife should be happy ! When I collected the car I did a 200 mile return trip, mostly motorway, and achieved 38mpg.
I expect the mpg to increase a little in the Summer when the temperature is higher.
For a 2.5 litre automatic, I'm far from disappointed with the fuel consumption. I'm sure you'll enjoy the cosseted luxury too, I guess your wife will hardly get a look in once you have a drive !!
Let us know what you get please.