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I've seen some many with the 255/35/19 at dealerships and they just look different. Keeping in mind that the dealership showroom cars are never lowered, but they just look less aggressive overall.
My dealer here displays both a IS250 lowered on F-Sport springs with F-sport 19" wheels and a non-lowered IS250 with F-Sport 19" wheels. It gave me a good perspective for comparison before I lowered my car on Eibach 350 springs.
For rear tire, the 275/30/19 will match the diameter of front tire and have a nice wide footprint in back. But I agree with above posters that this is just a personal preference, and the OEM sizing is most conservative.