...just buy 4 225/45s with no problems? I know the wider tires provide greater grip when accelerating and can disperse heat better, but realistically, why can't I just use the same front tire for all tires like every other vehicle. Makes for much easier maintenance, and lower costs as well, plus better tire rotation options. The engine is only 205HP so I think this whole wider tire thing is mainly for show and for the IS350 which has much greater HP. Give me your thoughts on using 225/45/18 at all wheel positions instead of just the front as it comes from the dealer.
I'll just add that the only difference between the 255 rear tires and the 225 front tires is the width of the tire. Both circumferences are identical.

if it is a front wheel drive, you should get along fine. if it is rearwheel drive, it will throw your speedometer off a little.

The suspension of your vehicle is tuned so that the wider rear tires prevent oversteer due to their wider contact patch. Installing a narrower tire at the rear gives you less traction than the vehicle was engineered for. Oversteer is when you spin. Spins are almost impossible to control and can lead to very serious accidents. The people who designed your Lexus have walls full of engineering degrees and tested the car for millions of miles before sending them off to the showroom. They do have some idea of what they are doing.

Stick with the factory tire sizes.

You are mistaken. they come with 225/40/18 up front. That is what gives the extra wheel gap up front. the 225/40/18 is 25.1" in diameter, while the 255/40/18 is 26.1" in diameter. that is 1 inch shorter up front. depending on the amout of sidewall you want, you could run 225/45/18 all around. if you go with the shorter 25.1" wheel in the rear, your speedometer will be off a little, but I am not sure by how much.

I had a inifnity g-35 before, and it came with the same tire/wheel set up, I put the 225's up front to get the same tire height all around.

you can check your tire diameter on tire rack, some tires may be up to 0.3" taller/shorter for the same size because all tire manufactures do not use the same techniques.