John,
In my experience of 2 1/2 years driving the 450h, I find that overall (brim to brim) 35mpg is the best I can get. The car is big, heavy, a 3.5 litre V6 and a superb place in which to be.
Yes, if you accelerate gently to your required speed, then back off so that the energy is flowing to your battery, you will not use fuel in the very short term, but OVERALL, my experience of 35mpg satisfies me.
Just enjoy the undoubted pleasure it can give.
Regards
John.