^ agree with aus301 about the crossover. seems pointless to me also.
but i have to say. i think you will notice a big difference with the optics aspecially the sharpness and colour.
i have a mate that does climbing photography and bought the 450 body with the 18-55 and 55-250.....
long story short when he upgraded to better lenses including L lenses the difference was noticable.
keep the 70-200 it's a great lens.