I would just get the 2.5 DP for now, headers really aren't a huge expense in comparison to a top-swap.
If you're going to go through that expense, might as well complete your exhaust work at the same time. As Herc said, you will see gains with the 3, but only on the top end, and you will probably feel the hit on the bottom end.
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'99 GTP, L36 bottom, XP cam, Headers, 3.29 gears, FWI, Powrtuner