My rule of thumb; if you add one tooth to the axle, and we're talking about running a 4.1 gear ratio, plus or minus a few, and we pick up 100 RPM at the end of the straight, we're going pretty close to the same speed.
As far as going over peak horsepower, it all depends on how fast the curve is going down past that peak point. If you go past peak horsepower, and you still have enough horsepower to keep accelerating, although you'll be accelerating slower, you're still going faster, and you have the advantage of the lower gear coming off the corner.