If you did not gas up to reset the fuel mileage after the service, the very slightly lower gas mileage may be due to the test drive they take after such a complete service. Computer memory may have also been reset and as someone mentioned already, the transmission has to learn your driving habits again. Aside from that, I'm curious why they changed the spark plugs so early, typically don't need change spark plugs on these engines until past 100,000 mi. The other question is then, what spark plugs did they use to replace the originals? The wrong spark plugs can affect fuel mileage.

 

I see no issue with the items you had serviced. As stated, it will take a while for the electronics to adjust. I recommend driving it normally and it will adjust over time, or you could take it back to the dealership and ask them to "re-learn" (reset using techstream) the engine and trans ecu.

Fill it up with 93 octane and drive it like you stole it.

 

None of what you performed improves mpg if there were nothing wrong with these things to begin with.

now you know.

 

I think you may have a misunderstanding of what can play a toll on fuel economy, as almost everything he mentioned has an effect on fuel economy no matter how small or large.
Anything that has a weighing factor on engine efficiency will lead to fuel economy influence, and as soon as you place a maintenance item into service it begins a life cycle or decline in efficiency. Efficiency is a loaded word in this instance because it includes so many factors, but all things are relative...