You can make a DA derailleur shift worse than Tourney if you adjust it badly. Like mpath said, go ahead and do it if it's real cheap, but otherwise, find a different shop.

I have 7800 shifters and 6600 derailleurs, and although it's nice, it's not worlds better than the all-105 setup I had on an earlier bike (dare I say, it's hard to tell any difference at all).