Somewhere around an additional 10-30 wheel HP and 15-20ft-lbs more torque with 3" compared to 2.5" (with tune) according to perrin's dyno testing.
And it doesn't "suffer" in lower rpm's as stated above, it has no loss of power or torque anywhere. It's just more noticeable in the higher rpm range.
http://blog.perrinperformance.com/3-...ults-and-tune/