Below info is taken from HONDATA's site.. this goes for the 2005-2006 ECU's as the 04-earlier ECU's are different..

This is what a Reflash gets you

• VTEC window from 4900 to 5800 rpm
• 8600 rpm Rev limit, increased from 8300 rpm
• Maximum torque gain 12 ft/lbs torque at 6000 rpm
• Maximum power gain 20 hp at 8000 rpm
• Peak torque increases by 6 ft-lbs
• Peak power increases by 8 hp
• Very wide flat smooth torque curve. 90% of peak torque available from 2200-8000 rpm.
• Fully dealer compatible - works with all Honda scan tools and equipment

We do not recommend the use of nitrous or forced induction for this reflash. For these applications use the K-Pro.

A VTEC "Window" was programmed in. This means the VTEC point is no longer fixed but varies according to manifold pressure (very closely related to throttle position) At sea level the VTEC point is 4900 rpm. At part throttle the VTEC switches at 5800 rpm. This allows you to cruise at 5300 rpm ( for example) on the lowcam and switch to the highcam just by giving more throttle. Also, as you rise in altitude, the VTEC point rises to 5800 rpm to reduce power loss after VTEC switchover.
You may feel a small dip at VTEC. Rest assured that even with this small dip you will have more power than before. You may also find the VTEC switch is smoother and quieter than before. Do not worry. VTEC is not missing; it is still operating.
This engine is sensitive to fuel quality and knock. With a higher octane fuel the ECU may advance the ignition slightly and make a little more power.
Overall the driving improvements to the 05/06 RSX mirror the 02-04 RSX reflash.