both cars have the same size gas tank. on the optima sx 2.0T I got 600 miles on a tank of gas ( highway) On the Cadenza I get just over 500 miles. In the city I'm getting about the same gas mileage. That's because I couldn't keep my foot out of the gas pedal on the optima. My wife liked the cooled passenger seat on the optima.
I didn't notice much turbo lag on the optima, and think it is much snappier on take off. turn off the traction control and put the shifter in the sport mode it is a lot more peppy than the cadenza. BUT who buys the Cadenza to be a sports car. The Cadenza's ride is a lot smoother than the optima and it has many more features.
Nice car, I got a white on beige, fully loaded. My co-worker's couldn't beleive that it was a Kia, especially considering it was parked next one of their 2012 535i's, and the Cadenza is larger, and has more room everywhere, along with more features. His car costs 20k more than mine, and has basically no features, no adaptive cruise control, no navi, blind spot, land departure, backup camera, rear sunshade, heated rear seats, he told me that he had to pay 950$ for the heated front seats.
They were all surprised, one of them thought it was a 7 series at first...I can't blame him though, it does look like it from an angle if you look at the back quarter panel.