Agree with Austin - look at what proposed OG was, what proposed effciency was, what your typical efficiency is (or if you don't track just go with theirs) and compare to your final gravity - if you add extract you could over power the yeast and end up with a beer that is sickly malty sweet and about 6.5% abv. Relax, let it finish out fermentation, bottle it, rest it for a couple weeks, and enjoy it. Another thought - when you tasted it, did you enjoy it? That's really all that matters, abv is only important for contests (style guidlines) or if people can't handle their drink.
I used to make high gravity beers for the sake of making high gravity beers, now I just make beer that tastes good, and a high gravity in the fall (porter, stout, wee heavy).
Relax and have a couple homebrews (sorry Charlie, one isn't enough for me).
Primary: IPA, Coffee Porter, Pale Ale
Secondary: pinot noir
On Tap: apfelwein, and more apfelwein
Next batch(es): Pilsen SMaSH
"Oh no, not again" - A Bowl of Petunias