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Sounds like something by either Jendra, Ross Wrock, or S Tar Kan on fanfiction.net
I don't remember reading that particular fic, but it might be on this list (EDIT: I read that as "this fic is a Peggy Sue", sorry. Maybe it's on the last section?): http://lira-snape.livejournal.com/3366.html
There are more lists here: http://lira-snape.livejournal.com/5672.html
At least it's somewhere to start looking, right?
Edited by MazzThanks for the suggestions! I knew it wasn't by S'Tar Kan (I'm a fan of a lot of-/familiar with all of- their fics), and - after looking through almost three-thousand bookmarks :P - I've found it: Taking Control by fake a smile - a bit more super!Harry and a lot Americanisms than I remember, but: success ;)

I'm looking for a Harry Potter fanfic; I can't remember much about it except that (somehow) Harry is gifted with more maturity (maybe?) (and the fic is not a Peggy Sue); unfortunately the only events from the fic I can remember are that Harry goes (muggle) clothes shopping, and goes on a not-date with a girl who works at the clothes shop; the only other feature of the fic I can remember are a few Americanisms (the girl asks Harry if he has a car when Harry (probably) wouldn't be old enough to hold a driving license in England, Harry refers to a building being a few blocks away, ...)
Not much to go on but I'd be grateful for any suggestions!
(Oh, and the fic isn't Forced Maturity.)