Originally Posted by jldc
If you add the extract late, it will be like using more hops. You may end up making the beer more bitter than you want - or you might like it better.
Do the math on the kit and you might find that the late extract addition may actually help you hit the IBU target. I recently did a BB porter kit, full boil. When I did the math to check what hopping adjustments I'd have to make to compensate for the full boil, I found that if I followed the directions to a tee, the bitterness would have been way low. I double checked with Beersmith and got similar results. If the kit had been dead on for it's 2.5 gal boil, it would have been over-hopped as a full boil or as late extract in a 2.5 gal boil.
Its obviously too late now, but if you do the math, you can check your hopping to see if late extract will help or hurt.