Probably just the cables stretching after the install. Properly set up and with good shifting technique on your part, the STI setup should shift smoothly.

As for "march this bike right back to the shop and demand that they re-adjust the derailleur?" How about bringing in a 6 pack and ASK! Most bike shop employees react much better to beer and courtesy than demands. Although your post implies you may have already burned some bridges. Good luck...