Hi Kaz,
Yes the bigger wheels add to mpg..but you will find as you put the miles on the fuel consumption should improve also the warm weather helps. Maybe all that extra equipment must add to the weight...I have only seen one other F sport in that colour in a garage in Galway.
This is my third IS, I had 200 for about four years and drove an IS 250 for the past nine years from new....didn't want to trade it but the tax and Mpg didn't make any logic in view to the cost of running the IS 300h...
Fin