Doubtful there is any reason to replace your injectors with 77k (or 180k) on them. They are probably fine as they are or you have complaints such as nailing. You could have the spray patterns checked, have them pop tested and reblanced (if needed) if you are really concerned about them. Mine have never been out of my engine.
I've never driven an entire tankfull around town so have no knowledge of city mileage. I typically get 31mpg on D2 (27-28 on B99) driving 110 miles a day, 80% of which is at 80mph and 20% at 5-20mph.
I'd look for other fuel leaks and then do a DP as it is a) cheap, b) may help and c) wont hurt. Heres a video how if you havent seen it yet.
http://www.peachparts.com/shopforum/showthread.php?t=113596