Nitromethane is much more than simply an octane reducer. You are correct that an octane improver is a cetane reducer, and vice versa, but that doesn't explain the complete function of nitromethane. As an example, Nitromethane (CH3NO2 - the numbers are actually subscripts) contains oxygen which in and of itself allows the higher power that is seen when using this as a fuel component. (Sort of like the racers using Nitrous Oxide in their engines for short bursts of additional power since it gives the engine increased oxygen) OK, you say, the chemical formula of amyl nitrate also contains oxygen, so why don't we see the same benefits and increased RPM? Simple. You are using Amyl Nitrate at 0.1% versus the use of Nitromethane at volumes ranging from 5 - 40%. Hence, a lot less oxygen is being added to your fuel.

Why are you seeing "...and improves accelerating and running but not increase rpm."? I would bet that if you actually measured the properties that you think are improved (very difficult without extremely finely tuned and calibrated specific equipment) that they really aren't, compared to the same fuel without 0.1% Amyl Nitrate. You aren't seeing an increase in RPM for the same reason - it isn't really there - but in this case you can measure it.

If your engines are running well on the mix you mention, they will likely run just as well with 0% Nitro FAI fuel.

By the way, the link you provided has just enough correct information as to make you believe everything that's said, and you shouldn't. For example, it is stated that "Nitromethane isn't added to sport fuels to provide power." This is simply not correct! A given glow engine will develop more power if it is given a higher dose of nitro - ask the folks that use 40% and higher on their race (plane and car and boat) engines, or the YS engines folks that suggest the use of 30% for optimum performance of their engines, or even ask yourself why it is that organizers of race events with controlled engine sizes provide the fuel to the competitors? It's to keep the competition level, so that one person doesn't use a high nitro content fuel to achieve a power - speed - advantage.