I just bought the 2004 pictured below, and it has all round 255zr2097y tyres. A quick call to my local General Tire place, indicated that they should be different sizes front and back.
Any comments about this? How does the car respond differently between the two states.
I have no problem reverting to the two sizes. I will not spend the extra money on Pirelli as I live in Houston, Texas, a place with no hills, two roundabouts and I am told, at least one road with a curve and where it snows once every four years.

On a funny note, my spell check wanted to change Pirelli to Zeffirelli but when I mispelled Zeffireli it wanted to substitute Schiaparelli.

 

AFAIK, one tyre size is used for front and back on the X350 saloons. My 2003 (UK) brochure however has no detailed specs for any of the variants, but 19" wheels are in the description of the XJR

 

Here are the official factory specs for the tyres;



From your pics it looks like you've got the 20" Sepangs, they should run 255/35/20 with a load rating 97 or higher.

Load of BS about the front & rear being different sizes, they should be all the same size.

 

The 20 inch variant seems to be an often chosen dealer option in the USA and common enough that most tire (tyre) places have information on both.
I am not usually a fan of large wheels and low profile tyres on US cars as we have more potholes in a mile of street than you have on the entire motorway system.