Well, 40mpg avg is not too bad.....your LT is heavier than my RS.
Did they change the air filter as part of the service? Its an absolute mare to do - so many don't.
The air temperature sensor could be the culprit - if thats awol it will keep the mixture rich. Worth checking.
Who did you use to set the throttle bodies up? As I said before if the butterflys have been tampered with, you will need some expert knowledge/experience to get them resynced.