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If you install the most recent Diablo tune and have a couple few mods you will hear and feel a little difference. I think that you will be lucky to get +10RWHP in my experience and that's if you have no cats, wide open exhaust and intake, UDP and other mods. Also, some say that it could take a while for your EEC to 'learn' the new curves so it may get a little better with time.
i'm guessing 6-7RWHP. that's what i was told when i was going to get chipped for my gears back in the day. hardly worth the money that you'd have to spend on 91+.