Just got back from a 1300 mile road trip up and down the south east coast on my 13 GS.
Good: Lux seating package ..outstanding 16 hour road trip and were the best seats ever.
Mileage: averaged between 28-29 MPG during the whole trip cruising 75-80.
love rear wheel performance.

Bad:
park assist sensor warning light kicked on the front. don't know why?
dash board computer screen takes a long time to boot up when starting.

 

Sounds like a great trip! Was there any salt/ice/grime on the front of the car? That would set off the sensors.

 

You sure you have a GS and not an ES? Seriously, I don't think I've ever gotten that high of mileage from my GS--other than a brief stint at 45 mph to find out the 'max' MPG the car is capable of.