I have no personal experience with Kumho tires......in the past on this site & others I belong to they are not very hightly rated....remember you only get what you pay for. ;)
You would have to get a set of 16" wheels, but if you found a set of OE Lexus 16" wheels....they should be 'plug & play' with no issues. The advantage of a larger wheel/tire would be that it would fill out the wheel well a little better & depending on the tire you select, there is a good chance you would get more overall longevity.....not so much better fuel economy.
Is it worth it? Depends on how you look at it. I am subsituting my ES 330 (17") alloy wheels for a set of ES 350 17" ers.....I like the design better personally......although to the untrained eye, most will not even notice the change.....but I will....and that's what matters.....as I have to have my cars a certain way & I prefer to 'personalize' it with OE wheels as opposed to an aftermarket set. B)