My '06 came from the factory with 16" wheels, and my speedo was off when I switched to 17" wheels. I don't remember by how much, but it was off.
So, to be exact:
225/55R16 = 25.7" diameter (80.8" circumference)
235/50R17 = 26.3" diameter (82.4" circumference)
So, the 17" is 0.6" bigger (2.3%). If the speedometer is perfectly right with the 16", then it will read 61 MPH when actually going 60 MPH, with the 17" wheels. Both of my LSes came from the factory with 17 inch wheels. Both read about 1 MPH faster than actual speed.
I could certainly be wrong, but I suspect that they used only one speedometer calibration for the LS.
It's the ABS module that provides the speed information to the rest of the car, and there doesn't seem to be any option on those for tire size. Fun observation, the VSS from the ABS is 10 times the speed in MPH, in Hz. For example, 80 MPH = 800 Hz.
BTW, (as I am sure you know) 16" wheels on an 06 is very rare. Almost all 06s were sports or premium sports. It must have been for a high snow location.