The thing about the IBR is that the Gov pays any unpaid 'subsidized' interest that accrued in your account. Lets say that you accrue $800 of interest from loans, and $250 of that is from subsidized loans (and $550 from unsubsidized loans). Then say your payment is $400, and $50 goes to pay subsidized interest, while $350 pays unsubsidized interest. Then the Gov will pay the $200 of subsidized interest that you didn't pay.
Now, say you have extra money, and instead of paying $400, you pay $800. I'm not sure if the Gov will still pay the $200 of subsidized interest that it would have paid if you only sent $400. There are two scenarios:
1) $600 goes to pay your sub/unsub interest, the Gov pays $200 for the sub interest, and your extra $200 goes towards your principle
2) $800 goes to pay your interest, and the Gov doesn't pay anything b/c you paid all the $800 interest that accrued for the month
If scenario #1 is true, then it makes sense to pay extra towards your student loans. If scenario #2 is true, then it's foolish to pay extra to your student loans and it makes sense to put the extra money elsewhere, such as retirement funds or a high-interest savings account and then after 3 years when the Gov no longer pays your sub interest, take out the money and pay off the student loans in one lump sum.
If anybody can shed some light on this it would be most appreciated