Completely agree with the other comments. I had an 05 300LE (the limited edition they threw everything into to get rid of old stock). I would average 21-24, mainly motorway driving in that. I did however get 33! mpg on one trip which I got driving with a feather foot - just to see what I could get out of it.
Just bought a 450H SEL Premier (09 plate) and though no motorway driving just shop trips or around Sheffield, the most I have had in 2 months is 24.5mpg. These are all the car readings, not measured readings. With this I have been driving carefully with the hybrid to see how I can squeeze miles out. Not scientific but it shows I am driving more carefully but not getting much more out of it than the old 300. I have a long run on weds/thurs so will do a measured calculation then.
Even though I had an 05 300 so I got the reduced VED (£280ish rather than £400+) the £135 on the 450H is welcome. The 400H will be a little more as it is higher CO2 emissions but still a saving on the 300.
My thoughts are, if you are expecting far better economy MPG wise it is not worth the change. BUT as others have said, the gearbox is FAR superior (IMO), smoother, quieter drive and as I got an SEL P, lots more toys I always wanted (and my Phone BT connection works better!) :D I changed because of the BT issues in the 300 with my phone and I wanted ACC. The hybrid was a plus, not a reason.
I actually preferred the 300 touch screen and interior but overall the 450h is a nicer ride and far more refined (with more toys)!