Do you have issues with how the car rides now? Is it too harsh? If you are fine with the current ride quality, I would see little reason to spend money swapping wheels and tires. Even if you are mostly ok with the ride quality, I would wait it out a bit to see how you adjust to it.
I myself have had some ride quality issues with the 20" wheels, but I am waiting to see how I adjust before I spend money on replacing the wheels and tires. Unless you can find someone to trade from a lower trim level or find some takeoffs for cheap, you are talking about thousands of dollars. Good tires for 18" wheels are going to run you close to or over $1000 on their own. If you go OEM wheels, you are probably looking at another $1000-$2000 on top of that. Even aftermarket wheels are going to be a couple hundred each. The one person on this forum that has swapped to smaller wheels only reported a small improvement in ride quality.
Personally, I would wait it out until I know I will not adjust and it is causing me to dislike the car, and I had evidence of people making the swap and reporting improved ride quality. While more sidewall should improve the ride, there are a lot of variables involved that could make that improvement minimal.