You won't nessecarily sacrifice gas milage or handling by leaving on the 16" rims/tires. If the Kumho's (or any other brand) are low budget ones, expect no miracles in terms as stated before. It's adviced you check this first.
Consider 17" wheels if you prefer a more firm ride, better handling and a bit more road noise. Bigger isn't nessecarily better, good tires make the difference.
It's perhaps a matter of taste and dineros. If you prefer the car to look & feel really stock, go OEM.
I appreciate all of the replies, now I am really confused and don't know what to do. The local Lexus dealer, Nalley Lexus (Galleria/Smyrna), let me use a loaner vehicle while I was having my vehicle serviced. The loaner vehicle was a 2010 GS 350 with 18 inch wheels. To be honest, the ride is better in my 2006 GS300 with the 16 inch rims/tires. I am wondering if the 17 inch tires will be better?