The Camry is designed for comfort and reliability. Giving it a more powerful engine wouldn't be of much benefit.

I'm not sure what the Optima is designed for. The general perception is that Koreans make 'me too' products. Toyota and Honda have D Segment sedans so the Koreans make D Segment sedans. It doesn't have to be better or have competitive advantages because they'll sell it cheaper.

The Optima GT finally has a competitive advantage in the D Segment but it's an old model which will be replaced soon. VW brought out their GT with the launch of the new Passat.