torque is more important that HP in normal driving, the 2016 kicks in at lower rpm, so very unlikely you will feel the difference on normal driving. Check out this video for full info on the vehicle along with his take on the engine changes.
I read in the optima forum that KIA stated they felt that 280hp was the limit on the motor and decided to decrease the hp and increase the compression allowing better tq thru the rpm range as a result better gas mileage and anticipated better reliability and longevity!
Always nice when the torque comes in at the lower RPM range and the engine runs on regular gas. I think the compression ratio is up slightly on the 2016 2.0 compared to the 2015 engine. Not sure if the 2015 engine needed 92-93 octane.