As stated you will get better utilization out of the hops, the IBU stated in the kit are most likely calculated using the recommended 1.5 gallon boil volume, so boiling more will up the IBU some. If you want even better utilization, you can also add a portion of the extract in the beginning of the boil and a portion of it in the last 20 mins. This will keep the gravity low during the bittering addition and get you more out of the hops. There's a thread around here with some great tips for extract brewing too. I'll post a link if I find it.
OP you might want to check this out:
Filled with mingled cream and amber I will drain that glass again. Such hilarious visions clamber Through the chambers of my brain -- Quaintest thoughts -- queerest fancies Come to life and fade away; Who cares how time advances? I am drinking ale today.
Last edited by RushN24; 02-05-2010 at 01:21 AM.
Reason: added link