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245/40R17 for most tires are square on 17x9. It's the popular setup for SCCA STX autox class, and most competitive cars in that class also happen to look pretty good too.
You may get a slightly larger contact patch with 255/40, but it's a taller tire than 245/40R17 and 215/45R17 so you'll lose some acceleration due to effectively taller gearing.
Even though I have a set of 245/40 on 17x9 because it's optimal for my autox class, I prefer the steering feel and quicker-feeling acceleration of a narrower 215/45 or 225/45 on 17x7.5 or 17x8 for daily street use. It also feels more fun.
FWIW, I think you're putting in way more thought than what's needed in saving a few pounds in your street-driven car's wheel and tire setup. If you're not trying to find tenths and hundredths of a second on a track or autocross, pick a wheel with a size and offset that looks good (to you) with your desired ride height and camber settings, get a tire that's suitable for what you're doing with your car, and just enjoy driving it.
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