This is going to become one of the biggest technical threads of your life when guys like TrinityGT, Triblkcobra, mrvax, and white99GT see this. Thats not bad though. Its just gonna get long so I figured Ide warn you.
Anways, its impossible to tell you how much rwhp is lost from a gear change. The average I hear is 5-8rwhp. Some maybe higher. Depending on the car and what steep of gear changer were talking. Its all physics in motion. I cant give the mathematical formulas for explanation. However, I do know the physical explanation in terms of centrifical force flow. To avoid getting technical, just think about the gears. The steeper the gear, the less the splines. The less the splines, the more power is lost across the gap of theese splines as the engery trasnfers from the one spline to the next across the ring & pinion gears in a centrifical manner. The power is needed to turn the gears but the further the gap in splines, the more the loss of power. Many people would assume then that steeper gears would slow you down just by that explanation. Not so. Remember, although power is being lost there are still less splines to wind. Therefore, the less time is needed for rotation of the ring and gear pinions. Thats why you gain in acceleration even though you lost rwhp. Thats just a basic explanation though. Im sure other guys here will have more " in-depth " explanations.