You may lose a few HP below 3000 RPM, but in reality, what does 4 HP at 2500 RPM really mean? Its not like you spend much time there when you floor it. If you need to go faster, downshift and floor it and the 2.5" exhaust will make the car faster than the 2.25" exhaust.As far as I've seen, with a 2.5" exhaust, yes you'll lose power/torque down low, but flow will be heavily increased in the upper-RPM ranges.
You are right. It may be better to go with a 3" if you go forced induction though.Also, (if I end up keeping my car) I'll be getting a 2.5" exhaust because it doesn't affect low-end that much and it keeps me open for a turbo/supercharger later down the line. Am I wrong for this?