There are a number of posts on this topic on horsepower, speed, and torque. This is my experience.
The Triumph engine I ran in 2007 produced 42 foot pounds torque at 3,500 rpm. The engine did not have much torque over 6,000 rpm so I ran a big 19 tooth countershaft sprocket. I ran just under 100 mph that year. I set the gearing so the engine was at about 6,000 rpm when I was going as fast as I could.
South Bay Triumph enlarged the intake valves 2mm and ported the head during winter 2007-2008. Now it had 50 foot pounds torque at 7,500 rpm. This is a 19 percent increase in torque and the bike felt about 19 percent more powerful. It was better, but not as good as I wanted. I was not happy. Lots of money spent for not much more get up and go.
I reset the gearing so the engine would be just past 7,500 when the bike was going to go as fast as it could. The countershaft sprocket size went from 19 to 18. The rear sprocket went from 33 to 34. This made a big difference. The bike went real fast. I scared myself. Life was good. The bike went just over 120 during the 2008 speed trials.
This made me do a lot of late night figuring and calculations. The gears and sprockets in a bike act as levers. The reworked engine, with its more high revving nature, was geared with a higher ratio (engine rpm/wheel rpm). It had a lot better leverage on the pavement and salt than the earlier low rev version.
This is my feeling, anyway, about why peanut sized high revving bikes go so fast. It is the gearing. Their high rpm engines allow them to be geared at higher ratios, and they use this better leverage to their advantage.
